Last updated February 24, 2014

Agent and Pathogenesis



Smallpox is caused by the variola virus. Variola virus classification:

  • DNA virus
  • Family Poxviridae, subfamily Chordopoxvirinae, genus Orthopoxvirus

Virion morphology:

  • Brick-shaped virion approximately 200 nm in diameter, 250 to 300 nm long, and 250 nm high (International Committee on Taxonomy of Viruses); about the size of a bacterial spore
  • Enveloped
  • Dumbbell-shaped core containing nucleic acid and surrounded by a series of membranes
  • Replicates in the cytoplasm of host cells, forming B-type inclusion bodies (Guarnieri bodies), unlike varicella or herpes viruses, which replicate in the nucleus

Genetic composition:

  • The genome is composed of a single, linear, double-stranded DNA covalently closed at each end.
  • Average genome has 200,000 base pairs (200 kbp) and is among the largest animal viruses.
  • A Web-based poxvirus genomic resource database is available (Poxvirus Bioinformatics Resource Center).
  • Comparative genomic analysis of 45 epidemiologically varied variola virus isolates from the past 30 years indicates low sequence diversity, suggesting little difference in functional genes. Analysis of viral linear DNA suggests that variola evolution involved direct descent and DNA end-region recombination events (Esposito 2006).

Variola viruses traditionally have been classified as variola major and variola minor on the basis of the severity of clinical illness caused by infection. Recognized variola minor strains include:

  • Alastrim
  • Amass
  • Kaffir

There are many viruses in the family Poxviridae with vertebrate host ranges that do not include humans; related viruses that can cause natural infections in humans include:

  • Other Orthopoxvirus species
    • Monkeypox virus
    • Vaccinia virus
    • Cowpox virus
  • OtherChordopoxviridae genera
    • Yatapoxvirus: tanapox virus, Yaba monkey tumor virus, and Yaba-like disease virus of monkeys
    • Parapoxvirus: Orf virus
    • Molluscipoxvirus: agent of molluscum contagiosum

Members of the Orthopoxvirus genus are genetically homologous and antigenically related (Jahrling 2007).

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The pathogenesis of smallpox involves the following steps (Fenner 1988, Henderson 1999: Smallpox as a biological weapon: medical and public health management):

  • The portal of entry for variola virus is usually through the oropharyngeal or respiratory mucosa; the virus also can enter through the skin, and rarely, through the conjunctiva or placenta (Fenner 1988).
  • The virus migrates rapidly to regional lymph nodes.
  • Asymptomatic viremia occurs on the 3rd or 4th day after infection, with further dissemination of the virus to spleen, bone marrow, and other lymph nodes.
  • Secondary viremia occurs by the 8th to 12th day after infection; this is followed by onset of fever and toxemia.
  • The virus localizes in small blood vessels of the dermis and oropharyngeal mucosa, leading to initial onset of the enanthem and exanthem, at which point (about day 14) the patient becomes infectious. The spleen, lymph nodes, kidneys, liver, bone marrow, and other viscera also may contain large amounts of virus (Breman 2002).
  • The development and evolution of skin lesions involves the following steps:
    • Dilatation of the capillaries in the papillary layer of the dermis occurs initially, followed by swelling of the endothelial cells in the vessel walls. Perivascular cuffing with lymphocytes, plasma cells, and macrophages can be seen.
    • Lesions then develop in the epidermis, where the cells become swollen and vacuolated; characteristic B-type inclusion bodies can be found in the cytoplasm.
    • The cells increase in size and the cell membranes rupture, leading to vesicular lesions.
    • Pustulation results from the migration of polymorphonuclear cells into the vesicle.
    • The contents of the pustule gradually become desiccated, leading to crusting or scabbing of the lesions.
    • Re-epithelialization and scarring occur as the lesions heal.
  • Death most commonly results from overwhelming toxemia, probably associated with circulating immune complexes.
  • Other long-term sequelae include secondary infections, encephalitis, blindness, and arthritis.

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Modes of Transmission
Environmental Survival
Historical Perspective


  • Before global eradication, the only reservoir for variola virus was humans. Variola virus does not persist or reactivate in individuals who have recovered from smallpox infection. Thus, no natural reservoir for the virus currently exists (Rotz 2010).
  • Stocks of variola virus have been retained in two World Health Organization (WHO)–approved collaborating centers: the US Centers for Disease Control and Prevention (CDC) in Atlanta and the State Research Centre of Virology and Biotechnology VECTOR (also known as the Vector Institute), Koltsovo, Novosibirsk Region, Russia (WHO 2001). All legitimate research with variola virus is now conducted within these two centers under biosafety level (BSL)-4 conditions, using a WHO-approved research agenda (Rotz 2010).
  • Some scientists are concerned that not all the smallpox preparations developed in the Russian bioweapons program can be accounted for and that unknown caches of variola virus may exist (Henderson 1998).

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Modes of Transmission

  • Variola virus is predominantly transmitted person-to-person via inhalation of large respiratory droplet nuclei (Fenner 1988). Transmission occurs most commonly among those with close face-to-face contact with an infected patient (particularly household contacts, since patients are usually ill enough to be confined to bed during the period of infectiousness).
  • Airborne transmission (via fine-particle aerosols) has been documented in two outbreaks that occurred in hospitals in the former Federal Republic of Germany (one in 1961 and one in 1970) (Wehrle 1970).
    • In the first outbreak, the index patient transmitted the virus to 19 persons, 10 of whom had no direct contact with the patient. The index patient had severe confluent skin involvement, ulcerative pharyngitis, and a barking cough.
    • In the second outbreak, the index patient transmitted the virus to 17 persons, none of whom had direct contact with the patient. The index patient had severe confluent skin lesions, severe bronchitis, and cough. Investigators noted that the relative humidity in the hospital was low (which may have facilitated survival of the virus) and that the design of the hospital set up strong air currents throughout the building (which may have facilitated dissemination of viral particles).
  • Although public health officials have traditionally considered spread via respiratory droplets as the primary mode of variola virus transmission, a recent review suggests that the role of airborne transmission may be greater than previously recognized. The author argues that smallpox should be considered a preferentially airborne infectious disease (anisotropic infection). Also, while severe disease can occur through different routes of transmission, a higher probability of infection and increased virulence is associated with infection via fine-particle aerosols (Milton 2012). 
  • Fomite transmission (eg, from clothing and bedding) has been reported (Dixon 1962, Kiang 2003). Contaminated blankets were used for intentional transmission of smallpox during the French and Indian War in the United States in the 1700s (Stearn 1945).
  • Transmission via direct contact with skin lesions and infected body fluids also has been recognized (Kiang 2003).

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  • The infectious dose is presumed to be low (10 to 100 organisms) (Franz 1997).
  • Most epidemiologic data have suggested that infectiousness in smallpox correlates with rash onset, with patients in the prodromal phase generally not considered infectious (Henderson 1999: Smallpox as a biological weapon: medical and public health management). This is distinct from varicella infection (ie, chickenpox), in which patients are infectious before rash onset. However, patients with smallpox should be considered infectious from the time of onset of fever, because virus is present in, and shed from, the oral lesions as they ulcerate during the 1 to 2 days of fever preceding rash onset (Breman 2002, CDC 2002: Smallpox response plan and guidelines).
  • Infectiousness is considered to be highest during the first week after rash onset, when lesions in the mouth ulcerate and release large amounts of virus into the saliva. Frequency of secondary transmission has been estimated (using a likelihood-based estimation procedure) as being highest from 3 to 6 days after onset of fever (Nishiura 2007).
  • The observed secondary attack rates among unvaccinated close contacts have varied from 37% to more than 88% (Arnt 1972, Heiner 1971, Kiang 2003, Rao 1968). The quantity of virus excreted in oropharyngeal secretions, the number of face-to-face contacts, and the extent of face-to-face exposure are considered key factors in determining infectiousness (Kiang 2003).
  • The average number of cases infected by a primary case is estimated at 3.5 to 6 (Gani 2001). This observation was consistent across analyses of outbreaks in isolated pre–20th century populations and in 30 outbreaks in 20th-century Europe. In these settings, herd immunity was low. This transmission estimate suggests that if smallpox is introduced in a population with little herd immunity, a rapid rise in cases would occur before control measures could be applied.
    • In 1972, delayed disease recognition led to a smallpox outbreak in the former Yugoslavia. A single unrecognized primary smallpox case led to 11 secondary cases. The secondary cases also were unrecognized, and within a few weeks, there was an outbreak of 175 smallpox cases resulting in 35 deaths and necessitating numerous control measures (Dembek 2007).
  • The period of communicability lasts until all the lesions have scabbed over and the scabs have fallen off. Viable viral particles can be detected in scabs (Fenner 1988, Wolff 1968); however, scabs are considered relatively noninfectious, since the viral particles are bound in the fibrin matrix of the scab.

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Environmental Survival

Variola Virus

  • Survival in the environment appears to be inversely proportional to temperature and humidity. In the pre-eradication era, smallpox had a higher incidence in climates in which winter and spring had low temperature and humidity.
  • Variola virus has been shown to remain stable for 2 to 4 months in scab material from smallpox patients (MacCallum 1957).
  • Variola virus apparently can persist on fomites (such as bed linens and clothing) for extended periods (months to possibly years) (Henderson 1999: Smallpox as a biological weapon: medical and public health management).

Vaccinia Virus

Although comparative data are not available, scientists have postulated that variola virus and vaccinia virus (the virus used in smallpox vaccines) likely have similar survival properties; therefore, information about survival of vaccinia virus is provided below.

  • Vaccinia virus released as an aerosol is almost completely destroyed in an atmosphere of high temperature (31°C to 33°C) and humidity (80%). In cooler temperatures and lower humidity, vaccinia virus aerosol survives as long as 24 hours (Henderson 1999: Smallpox as a biological weapon: medical and public health management).
  • A study showed that vaccinia virus is susceptible to germicidal ultraviolet (UV) light and that susceptibility increased with decreasing relative humidity when aerosolized (McDevitt 2007). However, one study showed that, when vaccinia virus is dried on a glass surface, 4% to 5% of the virions remain viable after exposure to germicidal UV light, suggesting the potential for fomite or environmental transmission following UV treatment (Sagripanti 2011). 
  • Studies with vaccinia virus suggest that the virus can survive on selected food and environmental surfaces as long as 2 weeks and in water at 4.5ºC as long as 166 days (Essbauer 2007).
  • In a case of severe eczema vaccinatum, household environmental samples were obtained after hospitalization of the patient. Findings from this study indicated that viable vaccinia virus can persist in the home for at least 10 days after being shed onto hard surfaces and clothing (Lederman 2009).

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Historical Perspective

Occurrence of Smallpox in the Pre-eradication Era

  • Smallpox likely originated in Egypt or India more than 3,000 years ago (WHO: Smallpox). Egyptian mummies dating from as early as 1500 BC showed characteristic pox-like skin lesions suggestive of smallpox.
  • Smallpox initially was introduced to the native populations of the Western Hemisphere by explorers from Europe and later by African slaves. The first recorded epidemic of smallpox in the New World occurred in 1507 on the island of Hispaniola (Fenner 1988). Eventually the disease spread throughout the hemisphere, with devastating consequences for many native tribes.
  • By the mid-1700s, smallpox was a major endemic disease throughout the world, except in Australia, where it was first introduced in 1789 and reintroduced in 1829.
  • Following the famous observations of Edward Jenner at the end of the 18th century, vaccination against smallpox using cowpox virus became a widespread practice in Europe and the United States. During the 19th century, cowpox virus was gradually replaced by vaccinia virus as the agent used in vaccination. During the first half of the 20th century, smallpox vaccination using vaccinia virus was widespread, particularly in Europe and the United States.
  • By the early 1950s, endemic smallpox had been eradicated from Europe, the Soviet Union, and North and Central America (Fenner 1988). Most of the outbreaks that occurred in Europe and North America after World War II were small and involved fewer than 50 cases (Bhatnagar 2006). However, the disease remained endemic throughout most of the developing world, with an estimated 50 million cases occurring each year (WHO: Smallpox).

Global Eradication of Smallpox

  • In 1958, a group of CDC epidemiologists responded to dual outbreaks of smallpox and cholera in East Pakistan. Controlling the smallpox outbreak posed numerous challenges, including the collapse of the local public health infrastructure. However, by the end of the campaign, 30 million Bengalis had been vaccinated, although another 20,000 had succumbed to the disease. This response helped to lay the foundation for and highlighted the challenges of smallpox eradication (Greenough 2011).
  • In 1959, the 12th World Health Assembly of the WHO passed the first resolution for global eradication of smallpox; however, it was not until 1967 that substantial resources were dedicated to the project.
  • The basic strategy of smallpox eradication included: (1) mass smallpox vaccination campaigns, and (2) surveillance and containment of outbreaks.
  • The last endemic case of smallpox occurred in Somalia in 1977, although a fatal laboratory-acquired infection was reported in England in 1978 (CDC 1978; CDC 1997: Smallpox surveillance—worldwide; Deria 2011).
  • After an extensive, sustained, international collaboration over 12 years, the International Commission for the Global Certification of Smallpox Eradication announced in December 1979 that smallpox had been globally eradicated (Fenner 1988). In May 1980, the WHO declared that smallpox had been eradicated. On the basis of the epidemiologic characteristics of smallpox, public health officials determined that a 2-year interval without recognized cases was needed before the disease could be declared eradicated. During those 2 years, extensive surveillance activities were carried out to "confirm the negative," which required tremendous effort by thousands of public health workers (Breman 2011).
  • The following epidemiologic features of smallpox facilitated global eradication (Fenner 1988):
    • Humans are the only natural reservoir for variola virus.
    • Vectorborne transmission of the virus does not occur.
    • The virus does not survive in nature for prolonged periods.
    • The full-blown clinical illness is easily recognizable, allowing for accurate clinical surveillance of the disease.
    • The infectivity of variola virus is relatively low (ie, transmission generally requires relatively close face-to-face contact except in uncommon circumstances), making it possible to effectively interrupt the chain of transmission.
    • Generally, only persons who develop the characteristic rash illness transmit the virus; subclinical illness is rare, and transmission from subclinical cases is not of epidemiologic importance.
    • No chronic carrier state of the virus occurs.
    • An effective vaccine exists.
    • The incubation period (10 to 12 days) is long enough for a vaccination-and-containment strategy to be effective.

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Use as a Biological Weapon

Historical Perspective
Impact of a Smallpox Release
Government Oversight

Historical Perspective

  • The unintentional introduction of smallpox to the Aztec Empire by Cortez in 1520 and its subsequent spread to the Inca Empire played a major role in the conquest of both empires (Martin 2007).
  • Smallpox was used as a biological weapon during the French and Indian War in the United States (1754-1767), when British soldiers gave the Native Americans blankets that had been used by smallpox patients (Stearn 1945). British forces also attempted to infect troops and citizens during the American Revolutionary War.
  • In 1972, more than 140 countries signed the Biological and Toxin Weapons Convention, which called for termination of all offensive biological weapons research and development and destruction of existing biological weapons stocks.
  • Despite participating in the 1972 convention, the former Soviet Union continued to expand its biological weapons program throughout the 1980s and early 1990s. During that time, the Soviet Union reportedly developed weaponized variola virus that could be mounted in intercontinental ballistic missiles and bombs for strategic use (Alibek 1999). Furthermore, the Soviet Union allegedly produced and stored weaponized smallpox in large quantities (20 tons per year) (Lucey 2009).
  • A 1971 outbreak of smallpox in Kazakhstan involving 10 people (3 of whom died) resulted from an open-air field test of 400 g of an enhanced weaponized strain of variola virus on Vozrozhdeniye Island in the Aral Sea (a top-secret Soviet bioweapons testing site and research facility) (Dembek 2007, Tucker 2002). The virus drifted 15 km downwind and infected the index case (Jahrling 2007).
  • Currently, variola virus is known to be stored in two facilities (at the CDC in Atlanta and at the Vector Institute, Koltsovo, Novosibirsk Region, Russia).
  • In the early 1980s, the WHO recommended that all existing stocks of variola virus held in other countries be either destroyed or shipped to one of the two WHO-approved collaborating centers. All countries reported compliance; however, there has been no systematic way to ensure that all countries actually did comply with the WHO recommendations (Henderson 2001). Also, there is no way to be certain that the virus has not fallen into the hands of rogue nations or potential terrorists (Henderson 1998).
  • On several occasions, the WHO has recommended that the remaining stores of variola virus be destroyed. However, in December 2001, the WHO Advisory Committee on Variola Virus Research recommended that existing stocks of the virus be retained so that various research goals can be achieved. The World Health Assembly has continued to authorize specific research projects that use the stocks, while acknowledging destruction of the stocks as its eventual goal (WHO 2001; WHO 2005; WHO 2011: Sixty-fourth World Health Assembly; WHO 2011: Smallpox eradication).
  • The WHO Executive Board placed the topic "smallpox eradication: destruction of variola virus stocks" as a substantive item for the sixty-seventh World Health Assembly, to be held in May 2014 (WHO 2013:  Smallpox eradication). In preparation for the assembly, the WHO Executive Board summarized the perspectives of two groups:
    • The Advisory Group of Independent Experts to Review the Smallpox Research Programme (AGIES) has determined that live variola virus is no longer needed for development of diagnostics for smallpox, for the development of vaccines, for sequencing the genome of additional variola virus isolates, for use in animal models, or for development of antiviral agents (AGIES 2013). The last review by AGIES determined that keeping live virus was necessary for use in animal models and for antiviral development.
    • The WHO Advisory Committee on Variola Virus Research determined that live variola virus was not necessary for the development of diagnostics or vaccines but that it was still necessary for the development of antivirals (WHO 2013: Advisory Committee on Variola Virus Research).

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Impact of a Smallpox Release

  • Smallpox is of concern as a biological weapon for several reasons: (1) much of the population is susceptible to infection; (2) the virus is infectious via aerosol, has a low infectious dose, and is transmissible from person to person; (3) the disease has a long asymptomatic incubation period and a high rate of morbidity and mortality; (4) very few treatments exist; (5) vaccine is not yet available for general use; and (6) past experience has demonstrated that introduction of the virus creates a great deal of havoc and panic (Henderson 1998, O'Toole 2002).
  • Additionally, the impact of smallpox on the general population would be greater than during the pre-eradication era because the prevalence of immunosuppressed individuals is higher and population mobility has increased dramatically (Jahrling 2007).
  • Aerosol release of virus (such as into an airport or subway system) would be the most efficient form of release and would likely result in the highest number of cases. Other possibilities include use of "human vectors" (ie, persons who have been deliberately infected with smallpox) and use of fomites (eg, contamination of letters sent through the mail) (Kiang 2003).
  • Several studies have used modeling to examine the impact of a deliberate release of smallpox virus. Using a stochastic model, investigators estimated that 100 index smallpox cases in a city of 2 million would result in 730 additional cases, assuming that control measures begin at 25 days after release and that the outbreak is controlled with ring vaccination and case isolation (Legrand 2004). Given the possibility of an intentional release of variola virus, ongoing global vigilance to rapidly detect any recurrence through accidental or intentional release is necessary (Breman 2003). Furthermore, even if all stocks of naturally occurring smallpox virus are destroyed, it is now possible to genetically engineer a similar viral agent in the laboratory setting. This capability requires that the medical and public health communities maintain smallpox preparedness into the foreseeable future (Bray 2004).

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Government Oversight

  • The Intelligence Reform and Terrorism Prevention Act, designed to improve efforts to fight terrorism, was signed into law on December 17, 2004 (Enserink 2005, US House of Representatives 2004). The legislation contained an amendment ("variola amendment") inserted at the last minute that imposes severe penalties for attempts to engineer or synthesize the smallpox virus. The amendment defines smallpox virus as any virus that contains more than 85% of the gene sequence of variola major or variola minor (US House of Representatives 2004). Penalties include fines of up to $2 million and prison terms ranging from 25 years to life (Enserink 2005, US House of Representatives 2004).

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Clinical Syndromes and Differential Diagnosis

Variola Major
Variola Minor
Vaccinia Virus Infection
Differential Diagnosis
Distinguishing Features Between Smallpox, Monkeypox, and Chickenpox
Determining the Likelihood of a Smallpox Diagnosis

Variola Major

Variola major is the most severe form of smallpox and can be further classified into five clinical types on the basis of differences in rash characteristics and density. The prognosis differs among the types (Fenner 1988). The clinical types are:

  • Ordinary (or classic) smallpox
  • Flat-type (or malignant) smallpox
  • Hemorrhagic smallpox
  • Modified smallpox
  • Variola sine eruptione

In the pre-eradication era, diagnosing smallpox and distinguishing its type took into account clinical illness pattern, epidemiologic considerations, and laboratory findings. Although there is some overlap between ordinary, flat-type, and hemorrhagic smallpox, their clinical and epidemiologic features are sufficiently distinct to warrant separate consideration (see below), particularly to enhance clinicians' awareness of the various clinical manifestations of what should be an extinct disease.

Modified smallpox was like ordinary smallpox but had an accelerated course and was a milder illness with fewer skin lesions and a low case-fatality rate; it was more likely to occur in persons with some immunity from past vaccination. Variola sine eruptione occurred in vaccinated contacts of cases and was characterized by sudden onset of fever, headache, and backache. Illness resolved in 1 to 2 days without development of a rash.

Case-fatality rates in the pre-eradication era for the various types of smallpox were high; however, such rates may be lower with modern medical management and intensive care. Recovery results in prolonged immunity to reinfection (Rotz 2010).

Images of smallpox rashes are available from the CDC (CDC: Smallpox Signs and Symptoms).

Ordinary (Classic) Smallpox

  • Ordinary smallpox was the most common type of variola major infection and accounted for at least 90% of cases in the pre-eradication era.
  • The case-fatality rate was usually about 30% in unvaccinated persons (range, 15% to 45%) (Fenner 1988). Death resulted from hypotension and toxemia (associated with circulating immune complexes).
  • The rash illness of ordinary smallpox is somewhat similar to varicella, although disease severity is greater (Henderson 1999: Smallpox: clinical and epidemiologic features).
  • The rash consists of firm, raised pustules that can be confluent, semiconfluent, or discrete.

Clinical features of ordinary smallpox are shown in the table below. A risk-evaluation algorithm can be found on the CDC Smallpox Web site to help clinicians determine if a patient with rash illness is at low or high risk of having smallpox on the basis of the clinical features of the illness (CDC: Evaluate a rash illness suspicious for smallpox).

Clinical Features of Ordinary Smallpox

Incubation perioda

—10-13 days (usually about 12 days)
—May be as short as 7 days and as long as 19 days


—Lasts 2-4 days
—Frequency of prodromal symptoms in one large case seriesb:
    ~Fever, 100%
    ~Chills, 60%
    ~Headache, 90%
    ~Backache, 90%
    ~Vomiting, 50%
    ~Pharyngitis, 15%
    ~Abdominal pain, 13%
    ~Diarrhea, 10%


—Enanthem on mucosa of mouth and pharynx usually begins about 24 hr before skin lesions appear (initially papular, then vesicular, then ulcerative over several days)
—First few skin lesions often appear on face ("herald spots")
—Lesions spread to trunk and proximal extremities and then to distal extremities
—Lesions prominent on face and distal extremities, including palms and soles, in centrifugal pattern
—Lesions initially maculopapular (days 1-2), then vesicular (days 3-5), then pustular (days 7-14); pustules gradually scab over by end of second week or during third week
—Vesicular lesions often have central umbilication that may persist into pustular stage, but as lesions progress they gradually flatten
—Pustules often described as "shotty" (ie, like hard, round foreign bodies embedded in skin)
—Lesions extend deep into skin, often are painful, and pitted scarring occurs as they heal
—Lesions may be discrete (relatively few in number), semiconfluent, or confluent
—Lesions generally progress at same rate with relatively synchronous onset
—In partially immune persons, clinical course may be much less severe and rash may be atypical with fewer lesions and more rapid healing (ie, "modified smallpox")

Laboratory featuresa

—Relative or absolute increase in lymphocytes may be noted
—Granulocytopenia may occur


—Massive amounts of subcutaneous fluid may accumulate during vesicular and pustular stages of rash, leading to severe fluid and electrolyte disturbances, including renal failure
—Massive skin desquamation can occur in cases of confluent disease; patients may clinically and metabolically resemble severe burn victims
—Viral bronchitis/pneumonitis occurs relatively commonly
—Other less common complications:
    ~Corneal ulceration (about 1% of cases) and/or keratitis (about 0.25% of cases) may cause corneal scarring and blindness
    ~Secondary bacterial infections (particularly skin and pulmonary infections)
    ~Encephalitis (0.2% of cases)
    ~Osteomyelitis or arthritis (about 1.7% of cases; usually in children)
    ~Orchitis (rare, 0.1%)

Case-fatality ratesd

—Overall case-fatality rate for ordinary smallpox, 15%-45%a
—Likelihood of death varies by type of disease (ie, confluent, semiconfluent, or discrete)c
—Observed case-fatality rates by type of disease among unvaccinated patients in one large seriesb:
    ~Overall rate, 30%
    ~Confluent disease, 62%
    ~Semiconfluent disease, 37%
    ~Discrete disease, 9%

aFenner 1988.
bRao 1972.
cKoplan 1979
dCase-fatality rates are based on historical data from pre-eradication era; such rates may be lower with modern medical management and intensive care.

Flat-Type (Malignant) Smallpox

  • Flat-type smallpox accounted for about 6% of cases in the pre-eradication era and occurred most commonly in children; illness was usually fatal.
  • The rash seen in flat-type smallpox involves flattened, confluent lesions rather than the characteristic firm pustules seen with ordinary smallpox.
  • Flat-type smallpox is thought to be associated with a deficient cellular immune response to the virus, although immunologic data are generally lacking (Henderson 1999: Smallpox as a biological weapon: medical and public health management).

Clinical features are shown in the table below.

Clinical Features of Flat-Type Smallpox

Incubation period

—Similar to ordinary smallpox (mean, 12 days; usual range, 10-14 days)


—Similar to ordinary smallpox (ie, fever, headache, backache, abdominal pain)
—Lasts 2-4 days
—Severe toxemia may occur

Rash illnessa,b

—Lesions develop slowly
—Lesions rarely progress to pustular stage and remain soft and flattened
—Lesions may be "velvety" to touch by 4th or 5th day
—Lesions often confluent
—Lesions and surrounding skin warm to the touch and tender to slight pressure
—If patient survives, lesions gradually disappear without forming scabs and without scarring
—Skin peeling or desquamation may occur as lesions heal

Laboratory features

—Similar to ordinary smallpox
—Relative or absolute increase in lymphocytes may be noted
—Granulocytopenia may occur

Case-fatality ratec

—Case-fatality rate 97% in one series involving 236 patientsd

aFenner 1988.
bDixon 1948.
cCase-fatality rates are based on historical data from the pre-eradication era; such rates may be lower with modern medical management and intensive care.
dRao 1972.

Hemorrhagic Smallpox

  • Hemorrhagic smallpox accounted for between 2% and 3% of cases in the pre-eradication era. In one series, 200 cases occurred out of 6,942 hospitalized patients (Rao 1972).
  • Illness was more common in adults, and pregnant women appeared to be at greater risk.
  • Hemorrhagic smallpox involved hemorrhages into the skin and/or mucous membranes. Early-onset and late-onset forms were described (Fenner 1988).
  • The pathologic features of hemorrhagic smallpox are consistent with disseminated intravascular coagulation (Mehta 1967, Mitra 1976).
  • As with flat-type smallpox, a defective immune response is suspected as the cause; however, immunologic data generally are lacking (Henderson 1999: Smallpox as a biological weapon: medical and public health management). Several studies have found lower antibody responses among patients with hemorrhagic disease compared with those with ordinary disease (Downie 1969, Sarkar 1967).

Clinical features are outlined in the table below.

Clinical Features of Hemorrhagic Smallpox

Incubation period

—Similar to ordinary smallpox (mean, 12 days; usual range, 10-14 days)


Early-onset form: illness onset sudden, with high fever, severe headache and backache, and toxemia; hemorrhages often noted by day 2
Late-onset form: illness begins with a typical prodrome, lasting 3-4 days

Rash illnessa,b

Early-onset form: generalized dusky erythema, petechiae, and ecchymosis occur soon after illness onset
Late-onset form: lesions begin as macules and develop into pustules; bleeding at base of skin lesions occurs

Hemorrhagic manifestations

—In both forms, bleeding may occur from mucosal surfaces
—Features in one series of nine patients with early-onset formc:
    ~Subconjunctival hemorrhage, 67%
    ~Hematuria, 56%
    ~Epistaxis, 33%
    ~Hematemesis and/or melena, 33%
    ~Hemoptysis, 33%
    ~Bleeding from gums, 33%

Laboratory featuresc,d

—Relative or absolute increase in lymphocytes may be noted
—Granulocytopenia may occur
—Features consistent with disseminated intravascular coagulation are common:
    ~Clotting-factor deficiency
    ~Prolonged prothrombin time

Case-fatality ratee

—In one series of 85 patients, case-fatality rate was 96%b
—Death usually occurs during the first week of illness

aFenner 1988.
bRao 1972.
cMitra 1976.
dMehta 1967.
eCase-fatality rates are based on historical data from pre-eradication era; such rates may be lower with modern medical management and intensive care.

Smallpox in Children

The clinical picture of smallpox in children generally is similar to that seen in adults. However, in one series of 100 cases among children in India, the frequency of various signs and symptoms varied somewhat from those classically described (Sheth 1971). For example, headache and backache were less common, whereas vomiting, conjunctivitis, and cough were somewhat more common. Signs, symptoms, and complications identified in that series are shown in the table below. Of the 100 patients, 66 had confluent ordinary smallpox, 25 had discrete ordinary smallpox, 6 had flat-type smallpox, and 3 had hemorrhagic smallpox. Overall, 34 children died (including all of those with flat-type or hemorrhagic smallpox).

The case-fatality rate in infants may be somewhat higher than in older children or adults (ie, >40%) (Fenner 1988). In one case series, the case-fatality rate for infants was 85% (Mazumder 1975).

Infection in pregnant women often leads to premature labor and death of the fetus (Fenner 1988). The overall case-fatality rate for pregnant women estimated from analysis of mid-20th century outbreaks was calculated to be about 34%. The proportion of miscarriage or premature birth was found to be approximately 40%, but no clear pattern was discernable. Premature birth was highest during the last trimester of pregnancy (Nishiura 2006) No clear congenital syndrome has been associated with smallpox infection in utero.

Signs, Symptoms, and Complications of Smallpox in Children

Headache (15%)
Backache (15%)
Retro-orbital pain (15%)
Prostration (75%)


Fever (100%)
Vomiting (83%)
Conjunctivitis (77%)
Hepatosplenomegaly (75%)
Hypotonia (75%)
Cough (71%)
Hoarseness (71%)
Edema (71%)
Delirium (64%)
Convulsions (7%)


Constipation (66%)
Bronchopneumonia (37%)
Alopecia (19%)
Osteomyelitis (4%)
Subcutaneous abscess (3%)
Diarrhea (2%)

Adapted from Sheth 1971.

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Variola Minor

Variola minor is a milder form of smallpox that is caused by distinct strains of variola virus. Variola minor was first recognized in the late 1800s; during the early 20th century, it was the most prevalent form of smallpox in the United States and Great Britain. The illness may be difficult to distinguish from variola major infection in partially immune persons.

Distinguishing Features of Variola Major and Variola Minor
Variola Major
Variola Minora,b


—Constitutional symptoms severe

—Constitutional symptoms tend to be mild

Rash illness

—Lesions often confluent or semiconfluent
—Rash evolves over 2-3 wk

—Lesions usually discrete
—Rash evolves over 1-2 wk


—Flat-type or hemorrhagic disease occurs more commonly (6% and 2%, respectively, in one large seriesc)

—Hemorrhagic disease rare (<0.5%)

Case-fatality rate

—May be high (15%-45%)

—Fatal outcomes rare (<1%)

aFenner 1988.
bKer 1967.
cRao 1972.

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Vaccinia Virus Infection

Vaccinia virus is a live agent that is used in smallpox vaccines. It is also used in recombinant vaccines to combat rabies in certain US regions. Naturally occurring vaccinia virus infections also have been reportd, notably in Brazil. The clinical illness associated with vaccinia virus is somewhat similar to smallpox (ie, a vesiculopapular rash), although exposure history can generally be used to distinguish between the two.

Vaccinia virus infections have occurred under the following scenarios:

  • Following smallpox vaccination or contact with a smallpox vaccination site in another individual (see Risk of Contact Vaccinia in the Vaccination section)
  • Following laboratory exposure (CDC 2009: Laboratory-acquired vaccinia virus infection—Virginia, 2008)
  • Naturally occurring (Assis 2012, Schatzmayr 2011)
  • Exposure to oral rabies vaccine (Dato 2009)

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Differential Diagnosis

Other rash illnesses, outlined in the table below, are included in the differential diagnosis of smallpox.

Differential Diagnosis for Smallpox
Distinguishing Features

Ordinary and Flat-Type Smallpoxa



See Distinguishing Features Between Smallpox, Monkeypox, and Chickenpox below

Human monkeypox

Monkeypox virus

See Monkeypox below

Naturally occurring vaccinia virus infectionb

Naturally occurring vaccinia virus

—Lesions tend to occur on the hands and forearms
—Cases generally have exposure to infected cattle (usually dairy cows)
—Occurs in certain areas of the world with endemic disease in cattle (such as Brazil)

Disseminated herpes zoster


—Usually occurs in immunocompromised hosts
—History of chickenpox


Staphylococcus aureus
Streptococcus pyogenes

—Lesions often pruritic and not painful
—Lesions focal and not usually disseminated
—Lesions not "shotty"
—Gold-colored crusted plaques are classic
—Lesions superficial and not embedded into dermis
—Constitutional symptoms generally absent or minimal
—Usually occurs in children

Hand, foot, and mouth disease


—Usually occurs in children <10 yr of age
—Has autumn seasonal pattern
—Lesions may be confined to hands and feet (although dissemination may occur)

Disseminated herpes simplex

Herpes simplex virus

—Usually occurs in immunocompromised hosts
—Lesions are vesicular and do not progress to pustules

Secondary syphilis

Treponema pallidum

—Rash generally does not include vesicular phase
—Lesions not "shotty"
—Constitutional symptoms relatively mild
—Lesions generally evolve slowly from macules to papules to pustules (often over several weeks)

Molluscum contagiosum


—Usually occurs in healthy children or HIV-positive adults
—In healthy adults, lesions generally occur in genital area
—Lesions are painless
—Constitutional symptoms generally are absent
—Lesions may persist for several months (or longer in immunocompromised patients)

Erythema multiforme major (including Stevens-Johnson syndrome)

Associated with various infectious and noninfectious processes

—Constitutional symptoms and rash usually appear at same time
—Rash evolves rapidly
—Bullae or "bull's-eye" lesions common
—Extensive mucous membrane involvement, including conjunctivitis, common

Drug eruptions


—Lesions generally not pustular
—History of drug exposure
—Fever may be present, but severe toxemia usually absent

Bullous pemphigoid


—Tense bullae characteristic
—Occurs most commonly in elderly
—Intense pruritus may be present
—Constitutional symptoms usually absent
—Peripheral eosinophilia may be noted

Other skin conditions


—Insect bites
—Contact dermatitis

Hemorrhagic Smallpox


Neisseria meningitidis

Rapid progression to shock and often death

Hemorrhagic varicella


Usually occurs in immunocompromised children

Rocky Mountain spotted fever

Rickettsia rickettsii

—Tick exposure history may be obtained
—Occurs April through May
—Most US cases occur in southeastern and south-central states


Ehrlichia chaffeensis
Erhlichia phagocytophilia

—Tick exposure history may be obtained
—Petechial rash uncommon
—Peripheral blood smear may show morulae in neutrophils of patients with human granulocytic ehrlichiosis

Septicemia caused by gram-negative bacteria

Various bacterial agents

—Underlying illness usually present

Abbreviation: VZV, varicella-zoster virus.

aOther rash illnesses (eg, measles, rubella, scabies, scarlet fever) also may be considered in the differential diagnosis, although the rashes caused by these conditions generally are not characteristic of smallpox.

bSchatzmayr 2011.

Adapted from CDC 2007: Acute, generalized vesicular or pustular rash illness testing protocol in the United States, Fenner 1988, Moore 2006.

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Historical Perspective in Africa

Human monkeypox is caused by monkeypox virus, which, like variola virus, is in the Orthopoxvirus genus. Monkeypox is similar to smallpox, but illness is generally milder. Recognized cases have occurred predominantly in western and central Africa. Pertinent historic information about monkeypox in Africa is as follows:

  • The illness in humans is similar to discrete or semiconfluent ordinary smallpox (Jezek 1987). A prodrome (fever, headache, backache) lasting 1 to 3 days occurs, followed by eruption of a smallpox-like rash that lasts 2 to 4 weeks. The incubation period appears to be 10 to 14 days, with a range of 12 to 16 days (Damon 2011).
  • Monkeypox cases tend to have prominent lymphadenopathy, which generally is not a feature of either chickenpox or smallpox (Arita 1985, Breman 1980, Jezek 1987). This can be an important distinguishing characteristic between the three conditions.
  • The first human case was recognized in 1970; since then, sporadic cases and outbreaks have been recognized in Africa, although the illness appears to be relatively uncommon.
  • Natural animal reservoirs in Africa include several squirrel species and forest-dwelling primates (Khodakevich 1988). Lagomorphs (rabbits, hares, etc) and other rodents in addition to squirrels, including prairie dogs, also appear to be susceptible to infection. The ecological requirements and geographic distributions have been identified, and these may support further field studies and guide public health intervention strategies (Levine 2007).
  • The case-fatality rate was 11% in one series of 282 patients (Jezek 1987) and was 3% in one outbreak involving 71 cases (CDC 1997: Human monkeypox—Kasai Oriental, Zaire, 1996-1997), suggesting that the illness is less severe than smallpox. In both investigations, all deaths occurred in children younger than 10 years old (who had not received earlier smallpox vaccination).
  • Person-to-person transmission has been demonstrated (Arita 1985, Breman 1980, CDC 1997: Human monkeypox—Kasai Oriental, Zaire, 1996-1997, Jezek 1986, Jezek 1988). Secondary attack rates of 7.2%, 7.5%, and 15% have been reported among household contacts who had not received prior smallpox vaccination (Arita 1985, Jezek 1986, Jezek 1988). These secondary attack rates are lower than those observed for smallpox and reflect the lower propensity for spread of monkeypox compared with smallpox. Person-to-person transmission has not been documented beyond four generations (Damon 2011).
  • A serologic survey conducted among a random sample of households in eastern Sierra Leone identified 11 individuals with high orthopoxvirus-specific IgG values out of a sample of 866. These individuals were under the age of 29, and therefore had not been vaccinated against smallpox. Results suggest that a low level of orthopoxvirus (most likely monkeypox) transmission is ongoing in this region, despite the absence of recognized clinical cases. Additional work aims to obtain viral isolates and determine the frequency of transmission (MacNeil 2011).

US 2003 Outbreak

In June 2003, an outbreak of monkeypox was recognized in the midwestern United States (CDC 2003: Multistate outbreak of monkeypox—Illinois, Indiana, Kansas, Missouri, Ohio, and Wisconsin, Reed 2004). Key findings from the outbreak are as follows:

  • Seventy-one cases were reported to the CDC; 18 (26%) patients were hospitalized but no fatalities occurred.
  • No exposed healthcare workers developed monkeypox symptoms, although one worker had serologic evidence of recent orthopoxvirus infection; that person had received smallpox vaccine during the previous year (Fleischauer 2005).
  • Thirty exposed persons received smallpox vaccine to prevent monkeypox; three reported rash within 2 weeks after vaccination, and one of these persons was confirmed as having monkeypox.
  • In one report, three exposed persons were identified who had no symptoms but had serologic evidence of recent monkeypox infection. All three had received smallpox vaccine in the past (13, 29, and 48 years earlier). These findings suggest long-term persistence of cross-protective immunity to orthopoxvirus infection following smallpox vaccination in these individuals (Hammarlund 2005).
  • The outbreak was traced to contact with infected prairie dogs; the prairie dogs became infected through contact with six species of African rodents (including Gambian giant rats, rope squirrels, tree squirrels, brushtail porcupines, striped mice, and dormice).
  • In November 2003, the Food and Drug Administration (FDA) and the CDC issued an interim final rule to prevent further introduction and transmission of the monkeypox virus (HHS 2003). This rule included one set of regulations administered by the CDC that prohibited importation of African rodents. A second set of regulations, administered by the FDA, placed restrictions on the interstate movement of several species of African rodents and on prairie dogs. In September 2008, the FDA removed its restrictions because they were deemed no longer necessary; the CDC ban on importation of African rodents remains in effect (FDA 2015).
  • The CDC has published guidance for autopsy and safe handling of human remains of monkeypox patients (CDC 2003: Interim guidance for autopsy and safe handling of human remains of monkeypox patients).

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Distinguishing Features Between Smallpox, Monkeypox, and Chickenpox

Smallpox and monkeypox are generally quite similar, although smallpox often is more severe and the case-fatality rate is higher. Early in the clinical course, smallpox or monkeypox may be mistaken for chickenpox if the clinical suspicion for orthopoxvirus infection is low. Also, smallpox in partially immune patients may be mild and may resemble chickenpox. An assessment of suspected smallpox cases referred to the CDC between 2002 and 2004 found that chickenpox accounted for more than half of the cases (Seward 2004). Distinguishing features of the three illnesses are outlined in the table below.

Distinguishing Features Between Smallpox, Chickenpox, and Monkeypox
Smallpox (Variola Major)a


Lasts 2-4 days, with high fever, headache, backache, and severe prostration; vomiting and severe abdominal pain may occur

Prodrome often absent; if present, it is mild and brief (ie, about 1 day)

Lasts about 2 days and is similar to that seen with smallpox

Distribution of rash

Begins on oral mucosa, spreads to face, then expands in centrifugal pattern (ie, most dense on face and distal extremities)

Begins on trunk and expands in centripetal pattern (ie, most dense on trunk)

Often begins on face and spreads in centrifugal pattern (although cases have been reported with centripetal pattern of spread)

Lesions on palms and soles


Almost never occur





Common (up to 90%)

Timing for occurrence of lesions

Generally emerge over 1-2 days and then progress at same rate

Occur in "crops" and may be at different stages of maturation at any given point in time

Lesions usually progress at same rate but may occur in crops (in about 20% of patients)

Evolution of lesions

Progress over several days from macules (day 1), to papules (day 2), to vesicles (days 3-5), to pustules (days 7 to about 14), to scabs (day 14 to about 20)

Progress quickly over about 24 hr from macules to papules to vesicles, then to crusted lesions

Progress in pattern similar to smallpox

Sensation associated with lesions

May be painful and only become pruritic during scabbing stage

Often intensely pruritic; not usually painful unless superimposed bacterial infection occurs

May be painful (although often milder than smallpox)

Depth of lesions

Extend deep into dermis and often cause pitted scarring

Superficial and generally do not cause scarring

Generally superficial (although pitted scarring can occur)

Duration of illness

14-21 days

4-7 days

14-21 days


Patients often appear toxic, and case-fatality rate may be as high as 50%

Patients often do not appear severely ill and illness is rarely fatal

Illness can vary in severity but often is mild and self-limited


Cases can be expected to occur in all age-groups; illness may be somewhat milder in adults over age 30 who were vaccinated as young children

Most cases occur in children; adults likely to be immune

Cases can be expected to occur in all age-groups; illness may be milder in people who have received smallpox vaccination

aIllness may be milder in patients with partial immunity; fever may be less common and fewer lesions may occur with more rapid healing.
bAlthough the prodrome may be milder in patients with chickenpox, a review of 932 chickenpox cases demonstrated that 7% to 17% of unvaccinated patients with chickenpox may meet the smallpox febrile prodrome criteria as put forth by the CDC (Moore 2004).

Adapted from CDC 2007: Acute, generalized vesicular or pustular rash illness testing protocol in the United States, Di Giulio 2004.

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Determining the Likelihood of a Smallpox Diagnosis

The likelihood of a smallpox diagnosis determines the appropriate laboratory testing and handling of specimens. Several years ago, the CDC developed the following criteria for determining the risk of smallpox (CDC 2007: Acute, generalized vesicular or pustular rash illness testing protocol in the United States). An important caveat to this algorithm is that it is not designed to detect the most severe and atypical forms of smallpox (Lucey 2009).

High risk for smallpox (when all three of the following features are present):

  • Febrile prodrome (occurring 1 to 4 days before rash onset) with fever greater than 101°F and at least one of the following:
    • Prostration
    • Headache
    • Backache
    • Chills
    • Vomiting
    • Severe abdominal pain
  • Classic smallpox lesions:
    • Deeply embedded in the dermis
    • Firm/hard
    • Round
    • Well-circumscribed
    • May be umbilicated
    • May be discrete, semiconfluent, or confluent
  • Lesions in the same stage of development (ie, on any one area of the body, all of the lesions are at the same stage, whether papules, vesicles, or pustules)

Moderate risk for smallpox:

  • Febrile prodrome (as outlined above under "High risk for smallpox") and at least one major smallpox criteria (classic smallpox lesions as described above or lesions in the same stage of development) or
  • Febrile prodrome and at least four of the five minor criteria:
    • Centrifugal distribution (lesions are more numerous on the face and distal extremities)
    • First lesions appeared on the oral mucosa/palate, face, or forearms
    • Patient appears toxic or moribund
    • Slow evolution of lesions from macules to papules to pustules over several days
    • Lesions on the palms and soles

Low risk for smallpox:

  • No viral prodrome or
  • Febrile prodrome and fewer than four of the five minor criteria outlined above (under "Moderate risk for smallpox")

A study indicated that physicians in the United States may be poorly prepared to diagnose smallpox. Only 36% of 178 physicians correctly answered 3 of 4 questions regarding smallpox and chickenpox differential diagnosis. In addition, only 17% indicated that they felt "comfortable" diagnosing smallpox, and 95% thought physicians needed more training in smallpox diagnosis (Woods 2004). Similarly, baseline knowledge assessment of 631 physicians in 30 internal medicine residency programs in 16 states and Washington, DC, showed that only 50.7% of participating physicians could correctly diagnose smallpox (Cosgrove 2005).

An algorithm developed by the CDC to rapidly evaluate patients for smallpox may prove useful. A prospective, multicenter study used the algorithm to classify 26,747 cases of rash or rashlike illness at emergency departments and inpatient units of 12 acute-care hospitals in six states. Eighty-nine patients presenting with acute generalized vesicular or pustular rash were determined to be eligible for the study, and 73 were enrolled. Physicians or study staff classified none of the 73 as being at high risk, 72 as low risk, and 1 as moderate risk of having smallpox. The discharge diagnosis for 55 of the 73 patients was varicella illness. Use of the algorithm did not result in misclassification of any patients as high risk for smallpox (Hutchins 2008).

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Clinical Laboratory Testing

Specimen Collection and Transport
Laboratory Biosafety
Biosecurity Information
Laboratory Response Network (LRN)
Tests for Detection and Identification of Variola Virus
Rapid Tests for Diagnosis of VZQ and HSV
Testing in Areas With Confirmed Smallpox
Inadvertent Discovery of Variola Virus in a Laboratory Specimen

Specimen Collection and Transport

The CDC has established algorithms for laboratory evaluation of patients with acute, generalized vesicular or pustular rash illnesses, based primarily on the likelihood of smallpox (CDC 2007: Acute, generalized vesicular or pustular rash illness testing protocol in the United States).

  • High risk for smallpox (see "Determining the Likelihood of a Smallpox Diagnosis" in the Clinical Syndromes and Differential Diagnosis section): Contact public health authorities immediately, before collecting specimens. Photograph patient's clinical presentation for uploading to public health departments and the CDC (use digital camera or scan film image). Initial testing will be performed by Laboratory Response Network (LRN) variola surge capacity laboratories in parallel with the CDC. Electron microscopy (EM) can be performed at the local facility, assuming BSL-3 containment is used for preparation of the grid. Variola virus should be ruled out prior to other testing, and results should not be released without CDC confirmation. The Federal Bureau of Investigation may initiate chain-of-custody for high-risk specimens.
  • Low or moderate risk for smallpox (see "Determining the Likelihood of a Smallpox Diagnosis" in the Clinical Syndromes and Differential Diagnosis section): Testing may be conducted at LRN laboratories and/or clinical laboratories with at least BSL-2 facilities. Testing for suspected cases of monkeypox or adverse vaccine reactions should be conducted at LRN laboratories. If varicella-zoster virus (VZV) diagnosis is questionable, laboratory testing should begin as soon as possible. Testing options include Tsank smear; direct fluorescent antibody (DFA) test for VZV and herpes simplex virus (HSV); PCR for VZV, HSV, and enterovirus; EM; and viral culture. If all results are negative, nonvariola orthopox infection, such as a vaccine-related reaction or monkeypox infection, should be considered. Contact the local LRN laboratory for testing. If nonvariola orthopox tests also are negative, reevaluate the patient and assess the need for additional dermatologic and histologic testing. If upon reevaluation the risk for smallpox is upgraded to high, switch to the high-risk protocol and contact local public health authorities.
  • Environmental samples: Testing of environmental samples is performed only at LRN reference laboratories under the direction of public health or public safety authorities.

It is likely that laboratories will receive specimens from patients with possible orthopoxvirus infections without being notified of risk level for smallpox. Theoretically, properly practiced universal precautions should protect the laboratory worker and community from accidental exposure.

Specimen Collection

  • If a patient is defined as high risk for smallpox (see "Determining the Likelihood of a Smallpox Diagnosis" in the Clinical Syndromes and Differential Diagnosis section), physicians should immediately contact their local or state health department for further instructions before collecting specimens.
  • The CDC has outlined procedures for collecting specimens from patients who may have smallpox (CDC 2002: Smallpox response plan and guidelines).
  • Only recently vaccinated (ie, within the past 3 years) personnel wearing appropriate barrier protection (ie, gloves, gown, shoe covers) should be involved in specimen collection.
  • If unvaccinated personnel must collect specimens, they should wear fit-tested N95 respirators and appropriate barrier protection. They also should have no contraindications to vaccination in case the diagnosis of smallpox is confirmed and vaccination is immediately required.

The following table outlines collection of laboratory specimens for the diagnosis of smallpox (variola) and smallpox vaccine (vaccinia)–associated infections.

Collection of Laboratory Specimens for the Diagnosis of Smallpox (Variola) and Smallpox Vaccine Virus Infection (Vaccinia)
Specimen Collectiona

Vesicles or pustules

—Sanitize the patient's skin with an alcohol wipe and allow to dry.
—Use scalpel (or 26-gauge needle) to open and remove top of vesicle or pustule; place skin of vesicle top into a 1.5- to 2-mL screw-capped or plastic tube, let dry.
—Scrape base of vesicle or pustule with blunt edge of scalpel or with wooden end of applicator stick or swab and smear scrapings onto glass or plastic light microscope slide. Allow to air dry for 10 min.
—Take another slide and touch it repetitively to open lesion using progressive movement of slide to make a touch prep. Allow to air dry 10 min.
—Store dried slides in plastic slide holders, using a different holder for each patient.
—If slide is not available, swab base of lesion with polyester or cotton swab, place in screw-capped plastic vial, break off applicator handle, and screw on lid (do not add transport medium to vial).
—If available, touch shiny side of 3 electron microscope grids to unroofed base of lesion and air dry; place in gridbox. Use varying degrees of pressure (minimal, light, and moderately firm) in application of each grid to unroofed lesion.
—Biopsy two vesicles with 3.5- or 4-mm punch biopsy kit; place one biopsy in formalin and one in 1.5- to 2-mL screw-capped container without added fluid.
—Draw 10 mL blood into plastic marble-topped tube or plastic yellow-topped serum separator tube; if plastic tubes not available, use equivalent glass tubes and package with styrofoam protector (Note: central line sample may be needed if peripheral blood draw is difficult because of sloughing skin in dense rash area).
—Swab or brush posterior tonsillar tissue and package in 1.5- to 2-mL tube, as above (do not add transport medium).
—Draw 5 mL blood into plastic purple-topped tube, gently shake tube to mix contents (if plastic not available, use glass as described above).

Scab lesions

—Sanitize patient's skin with alcohol wipe and allow to dry.
—Use 26-gauge needle to pry off at least four scabs.
—Place two scabs in each of two screw-capped plastic 1.5- to 2-mL vials.
—Obtain two biopsy specimens with a 3.5- or 4-mm punch biopsy kit; place one in formalin and one in 1.5- to 2-mL screw-capped container.
—Draw 10 mL blood into plastic marble-topped tube or plastic yellow-topped serum separator tube as described above.
—Collect swab of tonsillar tissue as described above.
—Draw 5 mL blood into plastic purple-topped tube as described above.

Nondermatologic specimens

Collect as appropriate, such as cerebrospinal fluid for postvaccinia encephalitis.

Autopsy specimens

—Ship frozen portions of skin-containing lesions, liver, spleen, lung, lymph nodes, and/or kidney.
—Collect formalin-fixed tissue from skin-containing lesions, liver, spleen, lung, lymph nodes, and/or kidney; package separately from frozen fresh tissue.
—Use plastic vials, bottles, or slide holders as primary container for all specimens.

aCheck with local Laboratory Response Network laboratory before collecting specimens. Specific collection recommendations change and may vary depending on local policies.

Adapted from CDC 2002: Smallpox response plan and guidelines, CDC: Specimen collection of smallpox (vaccinia) vaccine virus, CDC 2003: Current expectations for laboratory testing and adverse smallpox vaccine reactions, CDC: Emergency Preparedness and Response > Preparedness for All Hazards > Labs > Specimen Collection and Shipping > Infectious Agents: Specimen Selection, CDC 2003: Smallpox and vaccinia laboratory testing: a national training initiative.

Handling and Shipping

Storage and shipping conditions are as follows (CDC 2003: Current expectations for laboratory testing and adverse smallpox vaccine reactions; CDC: Emergency Preparedness and Response > Preparedness for All Hazards > Labs > Specimen Collection and Shipping > Infectious Agents: Specimen Selection; CDC 2003: Smallpox and vaccinia laboratory testing: a national training initiative; CDC 2002: Smallpox response plan and guidelines; CDC: Specimen collection of smallpox [vaccinia] vaccine virus):

  • For patients with high risk of smallpox, contact public health authorities before collecting, processing, or shipping specimens.
  • Keep all specimens away from direct sunlight.
  • For formalin-fixed specimens, electron microscope grids, and touch preparations, store and ship at room temperature.
  • For whole blood and other specimens shipped within 24 hours of collection, store and ship refrigerated (4ºC). (Note: Spin, separate, and freeze serum onsite if shipping is to be delayed.)
  • For serum and fresh biopsy material and other material potentially containing infectious particles if shipped more than 24 hours after collection, store and ship on dry ice (–20ºC to –70ºC).
  • Seal vials with parafilm to avoid pH changes from dry-ice vapors.


  • Package one sample per container.

Additional shipping information:

  • Guidelines have been published for packing and shipping of infectious substances, diagnostic specimens, and biological agents from suspected acts of bioterrorism (ASM 2012).

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Laboratory Biosafety

  • Laboratory safety practices associated with variola virus and other potential agents of bioterrorism have been reviewed elsewhere (CDC: Biosafety in Microbiological and Biomedical Laboratories, Sewell 2003). Biosafety level 4 (BSL-4) practices are required for working with variola virus.
  • Vaccination is not recommended for LRN sentinel laboratory personnel (CDC: Emergency Preparedness and Response > Preparedness for All Hazards > Labs > Biosafety > Vaccines). If a sample containing suspect smallpox virus is handled, vaccination within 3 days postexposure is considered effective.

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Biosecurity Information

  • Variola virus is classified under WHO risk group 4. In general, specimens or culture isolates that are reasonably suspected to contain variola virus must be transported as "infectious substances." The US Department of Transportation regulations and International Air Transport Association (IATA) rules require training of all individuals involved in the transport of dangerous goods, including infectious substances (DOT and IATA 2012). Chain of custody should be documented for material that may constitute evidence of criminal activity.
  • Variola major virus (smallpox virus), variola minor virus (alastrim), and monkeypox virus are classified as select agents select agents and therefore are regulated under 42 CFR part 73 (Possession, Use, and Transfer of Select Agents and Toxins), which was published in final form in the Federal Register in March 2005 and amended in October 2012 (HHS 2012). As specified in the Public Health Security and Bioterrorism Preparedness and Response Act of 2002, 42 CFR part 73 provides requirements for laboratories that handle select agents (including registration, security risk assessments, safety plans, security plans, emergency response plans, training, transfers, record keeping, inspections, and notifications). These requirements went into effect on February 7, 2003, and override earlier government requirements regarding possession and transfer of select agents. Effective April 3, 2013, variola major virus (smallpox virus) and variola minor virus (alastrim) will be considered Tier 1 agents and subject to additional security requirements (HHS 2012). Select agents are biological agents designated by the US government to be major threats to public health and safety. A current list of select agents is published on the CDC Web site under information about the Select Agent Program (CDC/APHIS 2008). In addition, the CDC has published additional guidelines for enhancing laboratory security for laboratories working with select agents (CDC 2002).

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Laboratory Response Network (LRN)

The LRN is a national network of approximately 150 laboratories. The network includes the following types of labs: federal, state, and local public health, military, food testing, environmental, veterinary, and international (located in Canada, the United Kingdom, and Australia) (CDC: Facts about the Laboratory Response Network, CDC: The Laboratory Response Network).

The LRN structure for bioterrorism designates laboratories as sentinel, reference, or national. Designation depends on the types of tests a laboratory can perform and how it handles infectious agents to protect workers and the public.

  • Sentinel laboratories, formally called "level A laboratories,"represent an estimated 25,000 hospital-based laboratories that have direct contact with patients. In an unannounced or covert terrorist attack, sentinel laboratories could be the first facilities to encounter suspicious specimens. These laboratories generally have at least BSL-2 containment capabilities. Sentinel laboratories use the ASM Sentinel Level Clinical Microbiology Laboratory Guidelines to rule out microorganisms that might be suspected as agents of bioterrorism (ASM).
  • Reference laboratories, sometimes referred to as "confirmatory reference," can perform tests to detect and confirm the presence of a threat agent. These laboratories ensure a timely local response in the event of a terrorist incident. Rather than having to rely on confirmation from laboratories at the CDC, reference laboratories are capable of producing conclusive results; this allows local authorities to respond quickly to emergencies. These are mostly state or local public health laboratories but also include military, international, veterinary, agriculture, and food- and water-testing laboratories. Reference laboratories operate with BSL-3 containment facilities that have been given access to nonpublic testing protocols and reagents. One of the roles of the LRN reference laboratories is to provide guidance, training, outreach, and communications to the sentinel laboratories in their jurisdictions.
  • National laboratories have unique resources to handle highly infectious agents and the ability to identify specific agent strains through molecular characterization methods. These laboratories also are responsible for methods development, bioforensics, and select-agent activity.
  • LRN variola testing laboratories are facilities selected by the CDC to conduct enhanced variola virus identification tests and handling procedures (CDC 2007: Acute, generalized vesicular or pustular rash illness testing protocol in the United States).

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Tests for Detection and Identification of Variola Virus

In the past, tests for the detection and identification of variola virus included culture on egg chorioallantoic membrane, tissue culture, and direct examination of vesicle or pustular material. Currently, PCR and EM are the methods of choice.

  • PCR-based methods:
    • PCR now plays a significant role in the rash-illness testing algorithm (CDC 2007: Acute, generalized vesicular or pustular rash illness testing protocol in the United States). However, because the worldwide prevalence of variola infection is zero, the positive predictive value of variola-specific PCR testing approaches zero if applied to cases that do not meet the CDC case definition for high risk of smallpox. Such testing is not recommended pre-event because of the potential serious consequences of false-positive results. The use of multiple methods and multiple specimens increases the cumulative positive predictive value of laboratory testing (CDC 2003: Smallpox and vaccinia laboratory testing: a national training initiative).
    • LRN assays
      • A real-time PCR method based on the DNA polymerase E9L gene has been developed, validated, and deployed for vaccinia virus. This assay will also detect cowpox virus and monkeypox virus. In North America a positive test is considered diagnostic for vaccinia virus unless medical or epidemiologic evidence suggests otherwise. With slight modifications to the fluorescently labeled probe, this assay can also be used to detect variola virus (CDC 2003: Smallpox and vaccinia laboratory testing: a national training initiative).
      • A generic orthopoxvirus assay that utilizes a conserved target is being used at LRN laboratories as a screening test (CDC 2003:Smallpox and vaccinia laboratory testing: a national training initiative, CDC 2007: Acute, generalized vesicular or pustular rash illness testing protocol in the United States).
    • Other PCR-based assays
  • EM: Negative staining is used to visualize the characteristic large brick shape and fine structure detail of poxviruses (CDC 2002: Smallpox vaccination laboratory support; Madeley 2003). In the past, EM was found to be successful at detection and general differentiation of viral particles in approximately 95% of patients with variola or monkeypox infections and 65% of patients with vaccinia infections. EM played an important role in the 2003 US monkeypox outbreak. A protocol is available online (CDC: Negative staining electron microscopic protocol for rash illness). Several characteristics of EM should be kept in mind:
    • Relatively rapid
    • Can distinguish orthopoxviruses from other viral agents
    • Cannot differentiate between variola and vaccinia viruses
    • May not be as sensitive as PCR-based methods

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Rapid Tests for Diagnosis of VZV and HSV

Laboratories that have at least BSL-2 containment facilities can perform rapid tests for diagnosis of rash illness in patients not considered at high risk for smallpox. Local or state LRN laboratories should be contacted for testing of specimens from patients with a moderate or high risk of smallpox (see "Determining the Likelihood of a Smallpox Diagnosis" in the Clinical Syndromes and Differential Diagnosis section). The most likely alternative agents are VZV and HSV; available rapid tests for these two agents include the following:

  • Cytology smears: Tzanck preparations stained with Giemsa or Papanicolaou stain can be used to differentiate VZV or HSV, each of which forms multinucleated giant cells, from smallpox, which does not. The testing is rapid, inexpensive, and relatively sensitive (Cohen 1994, Gershon 1999, Koranda 2004, Oranje 1986).
  • DFA: An assay by Millipore (Millipore) detects VZV and HSV simultaneously. A comparison study showed a sensitivity of 80% and specificity of 98.3% for HSV with same-day turnaround. A shell vial direct immunoperoxidase assay had a sensitivity of 87.6% and a specificity of 100%, and a turnaround time of 1 to 2 days. With VZV-positive samples, the DFA had a correlation of 87.1% with a cytospin DFA method (Chan 2001). DFA was instrumental in ruling out 3 of 4 cases with moderate risk for smallpox in the United States during 2002 (CDC 2003: Smallpox and vaccinia laboratory testing: a national training initiative).
  • EM: Although electron microscopes and operators experienced in viral recognition are not widely available, EM can play a role in the rapid diagnosis of rash illness (Madeley 2003).
  • Standard PCR methods: PCR has been shown to be more sensitive than immunofluorescence for detection of VZV (Bezold 2001) and significantly more sensitive than EM (Jain 2001). PCR has been shown to detect HSV and VZV effectively from Tzanck smears and vesicle fluid but less effectively from fixed-tissue specimens (Nahass 1995). FDA-approved methods are not yet available.
  • Real-time PCR assays: Rapid PCR assays using TaqMan and LightCycler technologies have been developed for HSV and VZV. These assays appear to have very high sensitivity and specificity (Aldea 2002, Espy 2000, Hawrami 1999, Koenig 2001, Nicoll 2001, Ryncarz 1999).

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Testing in Areas With Confirmed Smallpox

Once smallpox is confirmed in a geographic area, additional cases can be diagnosed clinically (CDC 2002: Smallpox response plan and guidelines). In such situations, laboratory resources will be used for specimen testing in the following cases:

  • Those in which clinical presentation is unclear
  • Those that will provide information about a potential source of exposure
  • Those that will facilitate law enforcement activities or case detection

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Inadvertent Discovery of Variola Virus in a Laboratory Specimen

Variola virus and other poxviruses grow readily on many cell lines such as Vero, HeLa, SF, and MRC-5. Accidental discovery of variola virus by a clinical virologist would constitute a danger to the laboratorians and could precipitate unintentional release to the community. The following features of variola virus in cell culture have been described in the older literature (Fenner 1988, Kato 1959, Marennikova 1964):

  • In human cell lines, variola virus tends to form "hyperplastic foci" as cells are pushed together by growing cells around them.
  • Within 24 to 48 hours, giant multinucleated cells form.
  • On staining, there may be a circular arrangement of nuclei around an eosinophilic part of the cytoplasm, often containing inclusions.
  • Within 72 to 96 hours, the number of giant cells increases, as does degeneration of the cell layer.
  • Variola virus can cause numerous inclusion bodies (Guarnieri bodies) in the cytoplasm of infected cells, which can be viewed after staining by Giemsa, modified silver stain, or other stains. Interpretation was difficult during times of smallpox occurrence and would be more difficult today.
  • If an unusual cytopathic effect is observed on any cell culture, especially involving giant cells, laboratory personnel should determine the suspected diagnosis for the patient before proceeding with identification. If the patient is at high risk for smallpox or at moderate risk for smallpox without alternate diagnoses, then the cell culture should be sealed, stored securely, and the local or state health department contacted for further instructions. Staining of cells suspected of harboring poxvirus is not recommended.

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Treatment and Postexposure Prophylaxis

Treatment for smallpox largely consists of general supportive measures:

  • Adequate fluid intake (difficult because of the enanthem)
  • Alleviation of pain and fever
  • Keeping the skin lesions clean to prevent bacterial superinfection
  • Several drugs have been evaluated in recent years for effectiveness against variola virus:
  • Cidofovir, adefovir dipivoxil, cyclic cidofovir, CMX001 (a lipid conjugate of cidofovir), and ribavirin have shown significant in vitro activity against variola virus (Franz 1997). Studies suggest, however, that patients may require a higher dose of cidofovir than is currently licensed for humans and may also require probenecid to ameliorate the uptake of cidofovir by renal tubular epithelial cells (McSharry 2009).
  • Arestvyr (formerly ST-246 and SIGA-246), is a low–molecular-weight compound that is active against multiple orthopoxviruses, including variola virus (Grosenbach 2008, Jordan 2008, Quenelle 2007, SIGA Technologies 2006, SIGA Technologies 2008, Yang 2005). The drug was used (along with vaccinia immunoglobulin (VIG) and cidofovir) in a case of eczema vaccinatum in a 2-year-old boy (SIGA Technologies 2007, Vora 2008). The child recovered fully after a 48-day hospitalization. The US government bought 2 million doses of Arestvyr for the Strategic National Stockpile (SNS) (SIGA Technologies 2013). Arestvyr is not currently licensed in the United States but can be used under an investigational new drug (IND) protocol (SIGA Technologies 2013).

Recent experience with progressive vaccinia following smallpox vaccination has raised questions about dosing and quantity of treatment agents stockpiled for responding to a smallpox incident. Current estimates likely underestimate the actual amount of treatment doses that may be needed if a smallpox incident occurred (CDC 2009: Progressive vaccinia in a military smallpox vaccinee—United States, 2009; Hayden 2011). In one case of progressive vaccinia (CDC 2009: Progressive vaccinia in a military smallpox vaccinee—United States, 2009), a patient received 341 vials of VIGIV, 73 days of Arestvyr (formerly ST-246) orally, 68 days of Arestvyr topically, and 6 weeks of CMX001 (Lederman 2012). This patient was monitored closely throughout treatment, in part because Arestvyr and CMX001 were being used under IND protocols. Because of this monitoring, the patient's treatment was modified when resistance to Arestvyr was identified, and treatment was continued for 2 to 3 weeks after the last detection of viral DNA. This intense laboratory monitoring was crucial in developing a treatment plan and led to a successful outcome for this patient.

On December 14 and 15, 2011, the FDA Antiviral Drug Advisory Committee convened to discuss the treatment of individuals with an established smallpox infection in the event of a malicious reintroduction of smallpox. The committee concluded that not enough information was available about these antivirals to provide recommendations to the FDA about their use under these circumstances at that time (FDA 2012).  

Vaccination given within 4 days after exposure can modify the course of disease and reduce mortality. See "Use of Vaccine for Postexposure Prophylaxis" in the Vaccination section.

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Historical Perspective
New Vaccines
Recommendations for Use of Vaccinia Vaccines
The US Smallpox Vaccination Program
Vaccination Schedule
Dosage and Route of Administration
Local Reaction to Vaccination
Contraindications and Precautions
Vaccination of Healthcare Workers
Use of Vaccine for Postexposure Prophylaxis
Use of Vaccine During a Smallpox Emergency
Smallpox Vaccination Clinic Implementation
Adverse Events Following Smallpox Vaccination
Risk of Contact Vaccinia
Treatment of Vaccine Adverse Reactions
Liability Issues Following Smallpox Vaccine Administration

Historical Perspective

  • The first efforts at smallpox vaccination involved a process called variolation, which was the deliberate cutaneous inoculation of variola virus via infectious material obtained from smallpox pustules of a patient with active disease (Fenner 1988). Variolation was practiced as early as 1000 AD in China and India and gradually spread around the globe.
  • In the late 1700s, Edward Jenner successfully used cowpox virus to vaccinate people against smallpox. Because this practice was safer and relatively effective, it rapidly gained wide acceptance and replaced variolation as the primary method of conferring protection against smallpox.
  • Over time, vaccinia virus gradually replaced cowpox virus as the agent used in smallpox vaccine. Vaccinia virus is genetically distinct from cowpox virus, although its origin remains unknown. It may have been derived from cowpox virus initially and modified over time through serial passage in laboratory cultures, or it may represent another orthopoxvirus that is now extinct in nature.
  • Until February 2008, the vaccinia vaccine available in the United States was a lyophilized preparation of infectious vaccinia virus (Dryvax, manufactured by Wyeth Pharmaceuticals, Inc., Marietta, PA). All lots of Dryvax vaccine expired on February 29, 2008, and programs that held supplies of Dryvax vaccine were instructed to destroy those supplies by March 31, 2008 (CDC 2008: Newly licensed smallpox vaccine to replace old smallpox vaccine).
    • Studies suggest that individuals vaccinated in the past with the Dryvax vaccine appear to maintain clinically detectable immunity against vaccinia for at least 20 years (Simpson 2007, Viner 2005).
    • Immune response was examined in persons participating in a longitudinal study on aging, including those with documented history of one or more smallpox vaccinations (n = 209) or a known history of smallpox infection (n = 8) (Taub 2008). Study findings indicated the following: Vaccination elicits both specific and neutralizing antivaccinia antibody levels that remain elevated for many years (up to 88 years in this study), multiple vaccinations achieved only marginally higher immune response levels than a single vaccination, and immune response levels are comparable in vaccinees and smallpox survivors.
    • It is not clear, however, whether a remote history of receiving at least one dose of smallpox vaccine will prevent infection or modulate disease severity if infection occurs.
    • Intradermal skin testing with inactivated vaccinia virus may be a simple and reliable method for predicting residual immunity to smallpox (Kim 2006).

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ACAM2000 (Sanofi Pasteur), is a live vaccinia virus smallpox vaccine that was licensed for use in the United States by the FDA in 2007 (FDA 2007). ACAM2000 has replaced Dryvax smallpox vaccine because of withdrawal of the Dryvax license. The vaccine is derived from plaque purification cloning from Dryvax.

A phase 2 clinical trial demonstrated that at a dose of 6.8 x 107 pock-forming units (pfu)/mL, ACAM2000 elicited a successful immune response in 94% of subjects, which was similar to the proportion of subjects (96%) who responded to Dryvax vaccine at a dose of 1.6 x 108 pfu/mL (Artenstein 2005).

A randomized trial was conducted to compare the safety and immunogenicity of ACAM2000 to Dryvax (as well as to another clonally derived, cell-culture manufactured vaccinia strain, ACAM1000). A dose of 1.0 × 108 pfu/mL was administered to healthy vaccinia-naive adults. All subjects had evidence of successful vaccination, and immune responses and adverse events were similar for the study groups (Frey 2009).

Unlike Dryvax, ACAM2000 expires 18 months after release from the SNS (CDC 2008: Newly licensed smallpox vaccine to replace old smallpox vaccine). Sanofi Pasteur has supplied more than 196 million doses of ACAM2000 to the US government for its SNS (Sanofi Pasteur). Currently, enough vaccine is available in the stockpile for all Americans (CDC: Frequently asked questions about smallpox vaccine).

Worldwide, stockpiles of smallpox vaccine amount to about 750 million doses, which is equivalent to approximately 10% of the global population (Lucey 2009).

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New Vaccines

One newer approach is to develop vaccines that incorporate attenuated vaccinia–derived viruses (Belyakov 2003, Poland 2003). These vaccines are referred to as third-generation vaccines (or beyond), and, because they use attenuated vaccinia viruses, they should have reduced rates of adverse reactions. Examples of third-generation vaccines include the following:

  • Imvamune (see below)
  • ACAM3000 modified vaccinia Ankara (MVA)
  • LC15m8 (see below)

Highly attenuated MVA vaccines are possible alternatives that are safer than Dryvax but may not be as immunogenic (Earl 2004, Earl 2008, Greenberg 2013, Jones 2008, McCurdy 2004, Parrino 2007, Slifka 2005, Vollmar 2006, Walsh 2013).

  • The SNS received all 20 million doses of Imvamune (an MVA vaccine) ordered from Bavarian Nordic of Copenhagen, Denmark, in 2007 (Bavarian Nordic 2013).
  • The US government has ordered an additional eight million doses to maintain the SNS (Bavarian Nordic 2013).
  • During an emergency, Imvamune is approved for use in individuals with HIV infection or atopic dermatitis, including children, pregnant women, and nursing women with these conditions (Bavarian Nordic 2012).
  • On Jul 31, 2003, the European Medicines Agency (DMA) authorized marketing of Imvanex (the European version of Imvamune) in the European Union (EU), allowing EU countries to buy this vaccine (EMA 2013).

LC16m8, which is licensed for use in Japan, is an attenuated cell culture–adapted Lister vaccinia smallpox vaccine missing the B5R protein. One study suggests that the vaccine is well tolerated, with similar reactogenicity as Dryvax (Kennedy 2011).

A number of other studies on possible candidate vaccines (such as cell-culture vaccines, subunit vaccines, and recombinant vaccines) have been published in the last several years, and research into new vaccines is ongoing. Additionally, various smallpox vaccines (first-, second-, third-, and next-generation) have been reviewed (Handley 2009, Kennedy 2009, Metzger 2009, Nalca 2010, Walsh 2011).

The regulatory pathway for approving these third- and fourth-generation vaccines is complicated. Well-defined animal models or correlates of immunity are not available for evaluating new smallpox vaccines (Traynor 2011). These vaccines will potentially be used in a world where vaccine- and infection-related immunity to orthopoxviruses is waning and the risk of emerging orthopoxvirus infections may be increasing (Golden 2011).

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Recommendations for Use of Vaccinia Vaccines

  • The current CDC strategy for use of the vaccine in outbreak control is outlined under Use of Smallpox Vaccine During a Smallpox Emergency below.
  • Routine smallpox vaccination in the United States stopped in 1972 for children and in 1976 for healthcare workers. Prior to 1972, smallpox vaccine was recommended for all children in the United States at 1 year of age. The US military vaccinated new trainees against smallpox until 1990, when vaccination was discontinued, but reinstated vaccinations in 2002.
  • Smallpox vaccination using vaccinia vaccines is currently recommended for certain laboratory workers:
    • Vaccine is recommended for those who directly handle vaccinia virus cultures, contaminated dressings or other infectious material, recombinant vaccinia viruses, or other orthopoxviruses that infect humans (eg, monkeypox).
    • Vaccine also is recommended for those who handle animals contaminated or infected with vaccinia virus, recombinant vaccinia viruses, or other orthopoxviruses that infect humans.
    • One report described an unvaccinated laboratory worker in whom ocular vaccinia developed while he was working with multiple strains of vaccinia virus (Lewis 2006). The worker was an immunology graduate student who was conducting research as part of a thesis project. This case highlights the importance of biosafety practices and vaccination among those who work with these viruses in the laboratory setting.
    • Following the accidental vaccinia virus infection noted in the previous bullet, laboratory workers in Pennsylvania who handled the virus were surveyed to evaluate their knowledge, attitudes, and practices regarding smallpox vaccination. Overall, 73% of survey respondents had been vaccinated during the last 10 years. Almost all respondents (96%) had received training on the risks of working with live vaccinia virus and the risks of smallpox vaccine; however, most respondents were more concerned about adverse outcomes due to vaccination than from accidental infection (Benzekri 2010).
    • Five cases of laboratory-acquired vaccinia exposures and infections were reported to the CDC from 2005 to 2007, underscoring the need for proper vaccination, laboratory safety, infection-control practices, and rapid medical evaluation of exposures among laboratory personnel (CDC 2008: Laboratory-acquired vaccinia exposures and infections—United States, 2005-2007).
    • In 2008, an additional laboratory-acquired vaccinia infection occurred in an unvaccinated laboratory worker at an academic institution in Virginia (CDC 2009: Laboratory-acquired vaccinia virus infection—Virginia, 2008).
  • In December 2002, the US smallpox vaccination policy was changed to include the following:
    • Required smallpox vaccinations for military personnel.
    • Smallpox vaccination for "smallpox response teams" involving civilian healthcare workers and public health staff likely to be involved in the initial care of any patients with smallpox or control measures. (See further information about smallpox response teams in the next section.)

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The US Smallpox Vaccination Program

To enhance preparedness in the event of a smallpox emergency, in October 2002 the Advisory Committee on Immunization Practices (ACIP) and the Healthcare Infection Control Practices Advisory Committee (HICPAC) recommended smallpox vaccination for persons designated by the appropriate bioterrorism and public health authorities to conduct investigation and follow-up of initial smallpox cases (ie, smallpox response teams). These teams were to include medical team leaders, public health advisors, medical epidemiologists, disease investigators, diagnostic laboratory scientists, nurses, personnel who would administer smallpox vaccines, and security/law enforcement personnel (CDC 2003: Recommendations for using smallpox vaccine in a pre-event smallpox vaccination program).

The recommendations made by the ACIP and HICPAC in 2003 did not address the appropriate revaccination interval for individuals who participated in the program. Thus, interim guidance on revaccination was issued by the CDC in 2008; it attempts to balance the smallpox threat and risk of exposure during an outbreak, the risk for adverse events in revaccinees (and their contacts), and the need for a sufficient number of protected first responders (CDC 2008: CDC interim guidance for revaccination of eligible persons who participated in the US civilian smallpox preparedness and response program).

According to the CDC, 39,608 persons were vaccinated through the program as of October 31, 2005, which is the last date that information is available on the CDC Web site (CDC: Smallpox vaccination program status by state). Less than 17% of smallpox vaccine doses distributed to states for healthcare workers had been used by mid-2005 (Bass 2007).

  • Reasons for nonparticipation in the program included the relatively low risk of a smallpox outbreak, risks associated with vaccination, hospital costs, and high rates of contraindications to vaccination (Kemper 2005, Wortley 2006).
  • Public health departments achieved significantly higher vaccination rates than did hospitals (Lindley 2006).

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Vaccination Schedule

  • Vaccination consists of a single dose followed by a booster every 10 years.
  • Revaccination every 3 years (CDC 2001: Vaccinia [smallpox] vaccine: recommendations of the Advisory Committee on Immunization Practices [ACIP], 2001) should be considered for persons who work with:
    • Non–highly attenuated vaccinia viruses
    • Recombinant viruses developed from non–highly attenuated vaccinia viruses
    • Nonvariola orthopoxviruses such as monkeypox

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Dosage and Route of Administration

Before smallpox vaccine is administered, patients should be screened for contraindications and provided with educational information about the vaccine (CDC: Smallpox pre-vaccination information packet).

The vaccine is administered using a droplet of the vaccine applied to a bifurcated needle. This procedure is detailed by the CDC (CDC: Smallpox vaccination method, CDC: Smallpox vaccine administration video).

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Local Reaction to Vaccination

Upon primary vaccination, all recipients experience a local reaction to the vaccine. A typical reaction occurs in the following sequence (CDC: Smallpox vaccination and adverse events training module, CDC 2003: Smallpox vaccination and adverse reactions: guidance for clinicians):

  • At 3 to 5 days after vaccination, a red papule appears at the vaccination site.
  • By day 5 to 8, the papule becomes vesicular, then pustular, and reaches its maximum size at 8 to 10 days. The Jennerian pustule, which contains turbid fluid, is whitish, umbilicated, multilocular, and surrounded by an erythematous areola.
  • The pustule eventually dries, leaving a dark crust that normally separates 14 to 21 days after vaccination.
  • A pitted scar usually remains after the scab separates.
  • Regional lymphadenopathy and fever are common.
  • Response to the vaccination should be evaluated 6 to 8 days after vaccination to ensure that a typical reaction has occurred (ie, a take).
  • A mouse model has demonstrated that vaccination initially (up to 4 days) induces infiltration of macrophages at the site, followed by granulocytes and lymphocytes. After that time, a large recruitment of CD4+ and CD8+ T cells occurs (Jacobs 2006).

Cutaneous reactions to subsequent vaccinations are weaker and manifest a range of the local reactions described above. Vaccinated persons in whom a typical reaction to the vaccine develops are considered to be protected against smallpox, since more than 95% of such people have been shown to have increased antibody titers following vaccination.

Traditionally, palpable inflammation or a pustule (ie, a local response) was thought to be an essential marker for successful revaccination. However, a study of 80 soldiers who participated in the 2002 Israeli smallpox revaccination campaign demonstrated that soldiers who had no local response to revaccination were successfully revaccinated against smallpox when assessed by immunologic markers. In the study, 40 subjects who developed a local response ("clinical take") were individually matched for age, sex, and smallpox vaccinations with subjects who did not develop clinical take. Immunologic evaluation 2 years after revaccination showed that both groups responded equally well to the revaccination. The authors concluded that during a mass vaccination campaign, success assessment is not needed for revaccinated individuals who do not have a clinical take (Wiser 2011).

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Contraindications and Precautions

Vaccinia vaccine for pre-exposure use is contraindicated for the following groups (CDC 2003: Recommendations for using smallpox vaccine in a pre-event smallpox vaccination program):

  • Persons who have ever been diagnosed as having eczema or atopic dermatitis (even if the condition is mild or not currently active), because the risk of eczema vaccinatum is increased
  • Persons with other acute, chronic, or exfoliative skin conditions disruptive of the epidermis (eg, burns, impetigo, chickenpox, contact dermatitis, shingles, herpes, severe acne, psoriasis) until the condition resolves
  • Persons with conditions causing immunodeficiency (eg, HIV infection, leukemia, lymphoma, generalized malignancy, solid-organ or stem-cell transplant, agammaglobulinemia or other hereditary immunodeficiency, autoimmune disease); additional information on smallpox vaccination in patients with organ transplantation is available (Dropulic 2003), and a new vaccine has been developed for use in immunocompromised individuals (Bavarian Nordic: Imvamune, Dorrell 2007)
  • Persons receiving treatments that cause immunodeficiency (eg, alkylating agents, antimetabolites, radiation, corticosteroids, chemotherapy agents, organ transplant medications); persons who are taking or have taken high-dose corticosteroids should not be vaccinated within 1 month of completing therapy, and persons treated with other immunosuppressive drugs within the last 3 months should not be vaccinated
  • Pregnant women, because vaccination may result in stillbirth or death of the infant shortly after delivery
    • Before vaccination, women of childbearing age should be asked if they are pregnant or intend to become pregnant in the next 4 weeks—those who say yes should not be vaccinated (for more information on smallpox vaccine and pregnancy, see: CDC: Smallpox vaccination information for women who are pregnant or breastfeeding, CDC 2003: Smallpox vaccination and adverse reactions: guidance for clinicians, Nishiura 2006).
    • In a smallpox bioterrorism emergency, pregnant women at high risk of exposure should be advised to be vaccinated, since the risk of death and serious illness from smallpox in that situation would likely outweigh risks to the fetus from fetal vaccinia (Cono 2006).
    • A study of 376 women enrolled in the National Smallpox Vaccine in Pregnancy Registry showed that women vaccinated during pregnancy did not have higher-than-expected rates of pregnancy loss, preterm birth, or birth defects compared with those in healthy referent populations. Most of the women in the registry (77%) were vaccinated near the time of conception, before results of a standard pregnancy test would have been positive. No cases of fetal vaccinia have been identified (Ryan 2008: Pregnancy, birth, and infant health outcomes from the National Smallpox Vaccine in Pregnancy Registry, 2003-2006).
    • A retrospective cohort study employing information from Department of Defense databases examined outcomes among 31,420 infants born to active-duty military women during 2003-04 (Ryan 2008: Evaluation of preterm births and birth defects in liveborn infants of US military women who received smallpox vaccine). Analysis revealed that maternal smallpox vaccination during pregnancy was not associated with preterm or extreme preterm delivery. Maternal smallpox vaccination in the first trimester was not significantly associated with overall birth defects or of seven specific birth defects individually modeled.
  • According to the product labeling, smallpox vaccination is not recommended for women who are breast-feeding
  • Persons with hypersensitivity reactions to vaccine components, including polymyxin B sulfate, streptomycin sulfate, chlortetracycline hydrochloride, and neomycin sulfate
  • Persons under 18 years of age in nonemergency situations
  • Persons with household contacts who are immunodeficient, who have a history of eczema or atopic dermatitis (even if the condition is not currently active), or who have an acute, chronic, or exfoliative skin condition
  • Persons with physician-diagnosed cardiac disease, with or without symptoms or with three or more major risk factors for cardiac disease (hypertension, diabetes, hypercholesterolemia, heart disease at age 50 years in a first-degree relative, smoking) (CDC 2003: Supplemental recommendations on adverse events following smallpox vaccine in the pre-event vaccination program)

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Vaccination of Healthcare Workers

Management of the Vaccination Site

  • Vaccinated healthcare workers who are involved in direct patient care should keep the vaccination site covered with gauze or similar absorbent material, because vaccinia has been detected through at least day 21 after vaccination (Cummings 2008). This dressing should then, in turn, be covered with a semipermeable dressing that provides a barrier for containment of vaccinia virus to minimize the risk of transmission. Alternatively, products combining an absorbent base with an overlying semipermeable layer can be used to cover the site. The latter may be preferable, because it has been found to more effectively reduce viral passage to the external surface of the dressing (Savona 2007). The vaccination site should be covered with gauze, a semipermeable dressing, and a layer of clothing during direct patient care until the scab separates.
  • Vaccinated healthcare workers should be reminded that consistent hand hygiene with antimicrobial soap and water or an approved alcohol-based hand rub is critical for preventing contact transmission.

When to Recommend Administrative Leave for Vaccinated Healthcare Workers

  • Vaccinated healthcare workers do not need to be placed on administrative leave unless they: (1) are physically unable to work because of systemic signs and symptoms of illness, (2) have extensive skin lesions that cannot be adequately covered, or (3) are unable to adhere to the recommended infection control precautions.

Administration of Other Vaccines and Tuberculosis Testing

  • Smallpox vaccine may be administered at the same time as any inactivated vaccine and at the same time as other live-virus vaccines except varicella vaccine (varicella vaccine and vaccinia vaccine should be given at least 4 weeks apart). Parenterally administered live vaccines not given on the same day as smallpox vaccine should be given 4 or more weeks later.
  • Healthcare workers who are due to receive an annual tuberculosis skin test (PPD) should not receive the test until 1 month after smallpox vaccination.

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Use of Vaccine for Postexposure Prophylaxis

  • Immunity to variola virus generally develops within 8 to 11 days after vaccination. Since the incubation period for smallpox averages about 12 days, vaccination within 4 days after exposure may confer some immunity to exposed persons and reduce the likelihood of a fatal outcome.
  • Postexposure vaccination may be particularly useful for persons who have received a dose of vaccine at some point in the past, since such persons are more likely to mount an anamnestic immune response with revaccination (Henderson 1999: Smallpox as a biological weapon: medical and public health management).
  • Studies on the utility of postexposure vaccination have shown conflicting results. Examples of study findings include the following:
    • During an epidemic in Italy in 1946, 21 contacts who received vaccine within 5 days after exposure and in whom smallpox subsequently developed all had mild disease, whereas 31 contacts who were vaccinated 6 to 10 days after exposure had a case-fatality rate of 19% (Dixon 1948). These findings suggest a protective effect for postexposure vaccination only if vaccine is administered within several days after exposure.
    • In another study involving 34 patients vaccinated during the incubation period, 9 were vaccinated at least 8 days before illness onset; 4 (44%) of these patients died. Twenty-five were vaccinated 7 days or less before illness onset; 10 (40%) of these patients died (Mazumder 1975).
  • How late after exposure individuals can be vaccinated and not become ill is unclear.
  • There are no absolute contraindications to vaccination for an individual with high-risk exposure to smallpox. Persons at greatest risk of complications of vaccination are those for whom smallpox infection poses the greatest risk as well (Rusnak 2007).

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Use of Vaccine During a Smallpox Emergency

During the smallpox eradication campaign and during smallpox outbreaks in the past, a "ring vaccination" strategy has been followed. This approach is incorporated into the current CDC Smallpox Response Plan and Guidelines (CDC 2002: Smallpox response plan and guidelines). Ring vaccination essentially involves creating a circle of vaccinated persons around each case to interrupt the chain of transmission. The strategy involves the following steps:

  • Rapid identification and isolation of all smallpox cases
  • Identification and vaccination of contacts of smallpox case-patients
  • Monitoring contacts for development of fever and isolating them if fever occurs
  • Vaccination of household members of contacts if no contraindications to vaccination exist

Smallpox incubation period estimates from historical records have been used to determine the most appropriate quarantine period for persons exposed to smallpox. Results suggested that if the exposed individuals are quarantined for 18 to 23 days after the date of contact tracing, the probability of noninfection is 95% to 99% (Nishiura 2009).

Several studies have used modeling to inform preparedness and response planning for a deliberate release of smallpox.

  • In Sweden, four vaccine-based strategies (ring vaccination, targeted vaccination, mass vaccination, and ring vaccination combined with prevaccination of healthcare personnel) were evaluated using a smallpox outbreak simulation model. When effectiveness was measured by the total reduction in infections, the combined strategy was best; however, if efficacy was defined by the per-dose incidence reduction, ring vaccination was superior (Brouwers 2010).
  • A separate mathematical model developed by investigators in Great Britain confirmed that a locally targeted ring vaccination strategy was optimal (House 2010).
  • In a similar model in Great Britain, investigators compared two strategies: nationwide mass vaccination and vaccination targeted to geographic areas with cases. They determined that vaccination in areas with cases was more optimal than national vaccination in most situations. This strategy provided outbreak control and also limited the number of vaccine adverse reactions and deaths (Egan 2011).

In addition to ring vaccination, rapid voluntary vaccination of a large population may be required to:

  • Supplement priority surveillance and containment control strategies in areas with smallpox cases.
  • Reduce the "at-risk" population for additional intentional releases of smallpox virus if the probability of such occurrences is considered significant.
  • Address heightened public or political concerns regarding access to voluntary vaccination.

In the United States, large-scale voluntary smallpox vaccination would be initiated only in certain situations under recommendations from the Secretary of Health and Human Services (HHS). Wide-scale quarantine of communities likely would not be effective and therefore would not be recommended (Barbera 2001).

Guide B of the CDC Smallpox Response Plan provides detailed information on use of smallpox vaccine during an emergency situation (CDC 2002: Smallpox response plan and guidelines). Key points for vaccine use are outlined in the table below.

Vaccination Guidelines for Use of Smallpox Vaccine During a Smallpox Emergencya

Whom to vaccinate:
—Persons exposed to the initial release of the virus
—Persons who had face-to-face, household, or close contact (ie, up to 6.5 ft) with a confirmed or suspected smallpox patient after fever develops in patient and until all scabs have separated (no longer infectious)
—Healthcare personnel, public health personnel, first responders, law enforcement personnel, and others whose jobs put them at increased risk of exposure to smallpox
—Laboratory personnel selected for collection or processing of clinical specimens from confirmed or suspected smallpox cases
—Other persons with increased likelihood of contact with infectious materials from a smallpox patient, such as laundry or medical-waste handlers for a facility where smallpox patients are admitted
—Other groups whose unhindered function is deemed essential to maintaining basic community needs (eg, transportation, pharmacy) or response activities and who are not otherwise involved in patient-care activities but who have a reasonable probability of contact with smallpox patients or infectious materials (eg, law enforcement, emergency response, military personnel)
—Because of potential for greater spread of smallpox in a hospital setting due to possible aerosolization of virus from a severely ill patient, vaccination should be considered for those individuals present in the hospital during the time that a case was present and not yet isolated in an appropriate manner in a room with ventilation separate from other areas of the hospital

Persons with contraindications for vaccination but not in a situation likely to encounter a smallpox casea,b:
—Persons who have ever been diagnosed with eczema or atopic dermatitis, even if the condition is mild or not presently active
—Persons with other acute or chronic exfoliative skin conditions such as burns, impetigo, or varicella-zoster (shingles), herpes, severe acne, severe diaper dermatitis with extensive areas of denuded skin, or psoriasis should not be vaccinated until the condition resolves
—Persons with diseases or conditions that cause immunodeficiency
—Persons with inflammatory eye disease being treated with steroids
—Women who are pregnant or breast-feeding
—Persons with serious, life-threatening allergies to the antibiotics polymyxin B, streptomycin, tetracycline, or neomycin

aGenerally, those with vaccine contraindications who have face-to-face contact with a smallpox patient are at greater risk for development of smallpox than for complications from vaccination and should be considered for vaccination.
bPersons who have contraindications to vaccination and who are household members of contacts who receive vaccine should consider housing themselves separately from vaccinated household members, to avoid potential exposure to smallpox or inadvertent inoculation with vaccine virus, until their vaccination sites have healed. This will decrease risk of contact transmission through vaccination site.

Adapted from CDC 2002: Smallpox response plan and guidelines.

Guidelines for large-scale smallpox vaccination clinics can be found in annex 3 of the CDC Smallpox Response Plan (CDC 2002: Smallpox response plan and guidelines). This guide includes the following information:

  • Vaccine delivery and packaging logistics
  • Organization of a large-scale vaccination clinic
  • Logistics for IND administration of smallpox vaccine
  • Question-and-answer sheets on smallpox vaccine for the general public
  • Considerations for mass patient care

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Smallpox Vaccination Clinic Implementation

In general, smallpox vaccine clinics should follow standard operating procedures for administration of other vaccines. The CDC has provided information about operating postexposure vaccination clinics (CDC 2002: Smallpox response plan and guidelines, AHRQ 2004). In addition, the CDC has provided software programs (Maxi-Vac 1.0 and Maxi-Vac Alternative) that can be used by state and local public health officials to plan large-scale smallpox vaccination clinics with maximum patient flow-through. The programs can be downloaded for free from the CDC Web site (CDC: Download Maxi-Vac Version 1.0 and Maxi-Vac Alternative).

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Adverse Events Following Smallpox Vaccination

Serious adverse reactions to smallpox vaccination can occur (CDC: Adverse reactions following smallpox vaccination, Lane 2003). Images of smallpox vaccine reactions are available on the CDC Web site (CDC: Smallpox vaccination and adverse events training module). Historically, for every 1 million vaccinees, 1 to 2 deaths and hundreds of complications severe enough to require hospitalization have occurred. If the current population of the United States was vaccinated, several hundred deaths and tens of thousands of hospitalizations might result. This small but definite risk and the absence of endemic smallpox led to the halt in routine vaccination in the United States in 1972 (Kennedy 2009).

Well-documented adverse reactions from historical information (ie, before routine vaccination ceased in the United States in 1972) include the following:

  • Tenderness, erythema at the injection site and other localized reactions (including allergic reactions to tape adhesives), and secondary bacterial infections; local reactions greater than 10 cm are referred to as "robust takes"
  • Systemic reactions, including fever of at least 100°F, malaise, myalgias, local lymphadenopathy
  • Various dermatologic reactions, including erythema multiforme and Stevens- Johnson syndrome
  • Inadvertent autoinoculation of another body site; 25.4 to 529.2 cases per million primary vaccinees
  • Generalized vaccinia: vesicles or pustules appearing on normal skin distant from the vaccination site; 23.4 to 241.5 cases per million primary vaccinees
  • Eczema vaccinatum: localized or systemic spread of vaccinia virus; may be severe and can be fatal; 10.4 to 38.5 cases per million primary vaccinees
  • Vaccinia keratitis
  • Progressive vaccinia: progressive necrosis in the area of vaccination, often with metastatic lesions at other sites; can be severe and fatal; 0.9 to 1.5 cases per million primary vaccinees (Bray 2003)
  • Postvaccinial encephalitis; 2.9 to 12.3 cases per million primary vaccinees
  • Fetal vaccinia: occurs after primary inoculation of the mother during pregnancy; usually results in stillbirth or death of the infant soon after birth
  • Death: an estimated 1.1 deaths per million primary vaccinees, but this may depend on strains used for vaccines. An analysis of recent smallpox vaccination studies from the 1950s to the present in different countries has suggested that strain-related differences exist in death rates (eg, 8.4 deaths per million vaccinees for the Lister strain [Kretzschmar 2006]). The Bern strain (used in Germany and Austria in the 1950s) gave the highest estimate of deaths per million vaccinations (55) and the New York City Board of Health Strain the lowest (1.4).
  • Note: Myopericarditis was not noted as an adverse event in the past but has been documented more recently, as outlined below.

Adverse event rates tend to be much lower in revaccinees compared with primary vaccinees (Treanor 2006).

Myopericarditis and Cardiac Adverse Events

In addition to previously recognized adverse events, more recent experience with smallpox vaccine has demonstrated that myopericarditis is a potential adverse event. The etiology of postvaccination myopericarditis remains unclear, although it appears that the process is immunologically mediated rather than the result of direct viral infection of the myocardium. Epidemiologic studies have supported a causal relationship between myocarditis/pericarditis and smallpox vaccination (Neff 2008).

  • Among more than 1,200,000 military personnel vaccinated between December 13, 2002, and May 17, 2007, 140 cases of myopericarditis were identified, yielding a rate of approximately 117 cases per million vaccinees (DoD 2007, Eckart 2004); follow-up of 64 of these cases (63%) demonstrated that all patients had normalization of cardiac function at an interval of 32 +/- 16 weeks.
  • Among the approximately 721,600 military personnel stationed in Iraq or Kuwait from 2004 to 2008, 70 cases of pericarditis and 9 cases of myopericarditis were identified at the theater referral clinic, yielding an incidence of 7.4 and 0.95 cases per 100,000, respectively (Lin 2013). Eleven of these case-patients had received a smallpox vaccine 4 to 30 days before diagnosis, seven of whom were diagnosed as having myopericarditis a mean of 13.7 days post vaccination.
  • In addition to myopericarditis, 16 cases of ischemic cardiac events have been reported following smallpox vaccine administration to military personnel. Poland and colleagues (Poland 2005) conclude: "The available data do not support a causal association between ischemic cardiac events and receipt of smallpox vaccine; however, this possibility cannot be excluded."
  • Follow-up studies of 37,901 HHS smallpox vaccination program vaccinees revealed myocarditis or pericarditis in 21 vaccinees; 18 cases occurred among those who had been revaccinated. Myocarditis was mild with no fatalities, although 9 patients were hospitalized (Morgan 2008). Ischemic cardiac events occurred in 10 vaccinees (6 myocardial infarctions [2 resulted in sudden death] and 4 cases of angina), and dilated cardiomyopathy was observed for 2 vaccinees (Swerdlow 2008). The observed number of myocardial infarctions exceeded estimated expectations but remained within the 95% predictive interval.
  • The 31 vaccinees with confirmed nonfatal cardiac adverse events noted above were followed for intermediate to long-term health consequences (Sniadack 2008). After a median of 40 weeks from initial symptom onset, the health status for these individuals was as follows: 12 (38.7%) had no symptoms, 14 (45.2%) had returned to baseline state, and 5 (16.1%) had persistent symptoms but were stable. Also, 15 (48.4%) had developed at least one health-related quality-of-life change by the time of follow-up.

Other Severe Adverse Events

Other severe adverse events following smallpox vaccination in the current era, including generalized vaccinia, progressive vaccinia, and eczema vaccinatum, appear to be rare (Vellozzi 2005).

  • A case of progressive vaccinia was reported in 2009 for a military smallpox vaccinee diagnosed with acute myelogenous leukemia 15 days after vaccination. Although the patient eventually recovered, the clinical course was protracted and medical management was complex. Numerous therapeutic agents with different biologic mechanisms were used to treat progressive vaccinia in this patient, including VIG and ST-246. Furthermore, the quantities of therapeutics required were much greater than anticipated based on existing smallpox preparedness plans. These findings indicate that the estimated national supply of therapeutic agents and diagnostic resources required to care for smallpox vaccine adverse events should be reevaluated (CDC 2009: Progressive vaccinia in a military smallpox vaccinee—United States, 2009).
  • In 2007, a severe case of eczema vaccinatum was diagnosed in a household contact of a smallpox vaccinee (Vora 2008). The vaccinee had not exposed the patient, his 2-year-old son, prior to 21 days postvaccination; however, the vaccinee had a history of eczema in childhood. Furthermore, the child had atopic dermatitis. Prior to this case, eczema vaccinatum had not been reported among a US vaccinee or contact since routine smallpox vaccination was discontinued in 1972. Careful prevaccination screening is necessary to avoid such severe adverse events related to vaccinia virus.

Dermatologic Adverse Events

Several anecdotal reports have been published regarding dermatologic adverse events following smallpox vaccination. These events include:

  • Urticaria, exanthems, contact dermatitis, and erythematous papules (Greenberg 2004)
  • Focal and generalized suppurative folliculitis (Talbot 2003)
  • Hypersensitivity reactions (Bessinger 2007)
  • Benign dermal abnormalities, including exaggerated scarring, dermatofibroma, and nevus sebaceous (Waibel 2006: Smallpox vaccination site complications, Waibel 2006: Smallpox vaccination site reactions: two cases of exaggerated scarring and a brief review)

Malignant tumors at the smallpox vaccination site also have been reported, including basal cell carcinoma, malignant melanoma, squamous cell carcinoma, and fibrohistiocytic tumors (Waibel 2006: Smallpox vaccination site complications). Such lesions may occur as the result of scarring followed by malignant degeneration, although a causal relationship between vaccination and dematological malignancies has not been established.

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Risk of Contact Vaccinia

Contact vaccinia refers to transmission of vaccinia virus from newly vaccinated persons to susceptible unvaccinated contacts. Studies conducted during the 1960s found the risk of eczema vaccinatum in contacts of vaccinated individuals ranged from 8.7 to 20.0 per million primary vaccinations. The risk of other accidental contact infections ranged from 3.5 to 44.6 per million primary vaccinations (Lane 1970, Neff 1967, Neff 2002). These studies relied on appropriate recognition and reporting of contact cases; therefore, the data generated may not necessarily have reflected the true incidence of this condition.

A review article has suggested that the current risk may be somewhat higher than that observed in the 1960s for the following reasons (Neff 2002):

  • Most members of the population are not immune to smallpox (nonimmune persons shed more virus when vaccinated than those who have some immunity).
  • A higher proportion of the current population has atopic dermatitis (particularly children).
  • A higher proportion of the current population has HIV infection or other immunosuppressive condition.

Among the estimated 2.1 million smallpox vaccinations given in the United States from December 2002 to March 2011, 115 incidents of contract transmission of vaccinia were identified, for a rate of 5.4 cases per 100,000 vaccinees (Wertheimer 2012). The vast majority of these cases (104) involved secondary transfers. Three fourths of all cases (86) involved either household contacts or intimate partners of vaccinees. Wresting, sparring, basketball, and football accounted for 51% of the male cases. Median time from exposure of the contact to clinical onset was 6 days (range, 1 to 19 days). Fourteen cases required hospitalization, and one case was life-threatening. This review included peer-reviewed case reports and reports from the Vaccine Adverse Event Reporting System and the Defense Medical Surveillance System.

A 2013 report detailed secondary and tertiary transmission of vaccinia virus in sexual contacts (Shao 2013). In that report, a recently vaccinated member of the military had sexual contact with the secondary case-patient, who sought treatment for a painful perianal rash and lesions on the lip 9 days following the encounter. The lesions were positive for nonvariola orthopoxvirus. The secondary case-patient was hospitalized for 3 days and received VIGIV because of a history of eczema. The patient in the secondary case had sexual contact with a third person 3 days before seeking treatment. The tertiary case-patient developed lesions 2 days later and was eventually hospitalized and treated with VIGIV (because of a history of childhood eczema). The vaccinated military member remained free of symptoms, and the other two patients recovered.

Two 2011 reports detailed vaccinia transmission that occurred during contact sports. CDC researchers reported a four-person cluster linked to a martial arts gym in Maryland (Hughes 2011). Although the index case-patient was not identified, the researchers said he or she was likely a recent vaccinee. Also, New York public health officials described a newly vaccinated military member who wrestled two people in matches during which his vaccination site became exposed (Young 2011). The two other wrestlers became infected with vaccinia, and one of them subsequently infected a third wrestler in a match several days later. A household member of one of the first two contacts also contracted vaccinia. One of the wrestlers developed ocular vaccinia (Montgomery 2011).

Nosocomial spread of vaccinia virus can occur, as summarized in another review article (Sepkowitz 2003). However, among 27,700 healthcare workers vaccinated recently, no worker-to-patient transmission of vaccinia was documented (DoD 2007).

A 2004 study demonstrated that use of two occlusive dressings (an initial waterproof gauze-impregnated transparent bandage and an outer waterproof semipermeable bandage) to cover vaccination sites offered an excellent barrier against inadvertent transmission of vaccinia virus (Talbot 2004). Treatment of the vaccination site with povidone iodine ointment, beginning 7 days after transcutaneous smallpox vaccination, appears to reduce the risks of autoinoculation or contact spread (Hammarlund 2008).

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Treatment of Vaccine Adverse Reactions

Detailed information on treatment of adverse reactions to smallpox vaccine  can be found on the CDC Web site (CDC 2003: Smallpox vaccination and adverse reactions: guidance for clinicians). The 2003 CDC guidance outlines the use of VIG, cidofovir (a nucleotide analogue of cytosine), and topical ophthalmic antiviral drugs for ocular involvement. In more recent years, Arestvyr also has been used to treat adverse reactions associated with smallpox vaccination.

VIG is the primary product available to treat complications of vaccinia vaccination. VIG can be obtained by contacting the CDC Drug Service (CDC: Drug Service). VIG is a sterile liquid immunoglobulin G obtained from immunized donors. Two intravenous forms of VIG were approved by the FDA in February 2005 to treat patients in whom serious adverse reactions to smallpox vaccine develop, including the following (Wittek 2006):

  • Aberrant infections induced by vaccinia virus that include its accidental implantation in eyes (except in cases of isolated keratitis), mouth, or other areas where vaccinia infection would constitute a special hazard
  • Eczema vaccinatum
  • Progressive vaccinia
  • Severe generalized vaccinia
  • Vaccinia infections in individuals who have skin conditions such as burns, impetigo, varicella-zoster, or poison ivy dermatitis; or in individuals who have eczematous skin lesions because of either the activity or extensiveness of such lesions

Use of VIG is not recommended for isolated keratitis (as it may cause severe corneal opacities), erythema multiforme, and postvaccinial encephalitis (Rusnak 2007).

Cidofovir and Arestvyr may be used under an IND protocol to treat serious smallpox vaccine reactions (CDC 2003: Smallpox vaccination and adverse reactions: guidance for clinicians, CDC 2003: Medical management of smallpox (vaccinia) vaccine adverse reactions). The proposed dose is 5 mg/kg administered intravenously, one time, over 60 minutes; however, cidofovir should be administered in consultation with experts from the CDC or the Department of Defense. The CDC will supply cidofovir at no cost for use under the IND. Cidofovir will be released by the CDC in the following situations:

  • A patient fails to respond to VIG
  • A patient is near death
  • All inventories of VIG have been exhausted
  • Arestvyr is also available from the CDC at no cost for use under the IND

Off-label use of topical ophthalmic antiviral agents (trifluridine or vidarabine) has been recommended by some ophthalmologists to treat vaccinia infection of the conjunctiva or cornea. In rabbits, 1% topical trifluridine applied nine times in 10 days proved to effectively treat vaccinia virus keratitis. Use of VIG had no impact on clinical course; corticosteroids exacerbated the keratitis and should not be used (Altmann 2011).

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Liability Issues Following Smallpox Vaccine Administration

The Homeland Security Act of 2002 (Section 304) addresses issues regarding liability following smallpox vaccination (CDC: Smallpox questions and answers: Section 304 of the Homeland Security Act, US Congress 2002).

Key points from Section 304, which went into effect in January 2003, include the following:

  • Section 304 applies to:
    • Smallpox vaccine manufacturers
    • Healthcare institutions and public health agencies that distribute vaccine or that administer smallpox vaccination programs
    • Licensed healthcare providers or other individuals authorized to administer smallpox vaccine or other smallpox countermeasures under state law
    • Any official, agent, or employee of the above entities
  • Section 304 states that no claim for liability for injury or death attributable to smallpox countermeasures, including vaccination or other substances used to treat or prevent smallpox, can be brought against the entities or individuals who are covered under the section, unless gross negligence, recklessness, illegal conduct, or willful misconduct can be demonstrated.

A final rule for smallpox vaccine injury compensation was published in May 2006 in the Federal Register (HHS 2006). The rule clarifies eligibility standards, the process for requesting and receiving benefits, and other policies and procedures.

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Hospital Infection Control (Including Autopsies and Burial)

Isolation Precautions
Vaccination of Healthcare Workers
Cleaning and Disinfection of Environmental Surfaces
Disinfection/Sterilization of Reusable Medical Equipment
Laundry and Waste
Decontamination of Air Space
Autopsy Practices

Isolation Precautions

According to the CDC/HICPAC 2007 Guideline for Isolation Precautions, Airborne and Contact Precautions in addition to Standard Precautions should be implemented for patients with suspected smallpox (Siegel 2007). In addition, nonvaccinated healthcare workers should not provide care when immune healthcare workers are available. Initial cases of smallpox are likely to not be placed in Airborne Precautions, which could increase the number of second generation cases among healthcare workers (Milton 2012).

Airborne Precautions include:

  • Place the patient in a private room with negative air-pressure ventilation (minimum 6 air exchanges/hr for an existing facility or 12 air exchanges/hr for a new facility). Air pressure ventilation should be monitored daily.
  • Use external air exhaust (if possible) or high-efficiency particulate air (HEPA) filters if the air is recirculated.
  • Keep the door to the room closed.
  • Healthcare personnel caring for patients with suspected smallpox should wear a fit-tested NIOSH-approved N95 or higher level respirator for respiratory protection. Respiratory protection is recommended for all healthcare personnel, including those with a documented "take" after smallpox vaccination, because of the risk of a genetically engineered virus against which the vaccine may not provide protection or of exposure to a very large viral load.
  • These precautions need to be maintained until all scabs have crusted and separated (typically 3 to 4 weeks).

Contact Precautions include:

  • Place the patient in a private room if available.
  • If a private room is not available, place the patient in a room with a patient who has active infection with the same organism (ie, cohort patients with smallpox).
  • Wear gloves when entering the room, change gloves after having contact with infectious material, remove gloves before leaving the room, and immediately wash hands using an antimicrobial agent.
  • Wear a gown when entering the room if clothing will have significant patient contact; remove the gown before leaving the room.
  • Move and transport the patient for essential purposes only. If transport is necessary, a mask should be placed on the patient.
  • When possible, dedicate the use of noncritical patient-care equipment.

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Vaccination of Healthcare Workers

All healthcare workers caring for patients with suspected smallpox should be vaccinated immediately.

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Cleaning and Disinfection of Environmental Surfaces

No disinfectant products are registered by the US Environmental Protection Agency specifically for variola virus inactivation; however, according to the CDC, products that inactivate similar lipid or medium-sized viruses (such as vaccinia virus) are adequate for disinfection of variola virus (CDC 2002: Smallpox response plan and guidelines). These products include chemicals used on environmental surfaces for low- or intermediate-level disinfection and are outlined in the table below. High-level disinfectants or liquid chemical sterilants are not indicated for cleaning large environmental surfaces (eg, floors, walls, tabletops).

Chemicals Used on Environmental Surfaces for Low- or Intermediate-Level Disinfection
Minimum Concentration to Achieve Inactivation

Ethyl alcohol


Isopropyl alcohol


Benzalkonium chloride

100 ppm

Sodium hypochlorite

200 ppm




75 ppm

Abbreviation: ppm, parts per million.

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Disinfection/Sterilization of Reusable Medical Equipment

Standard disinfection/sterilization methods approved by the FDA for medical instruments and devices are considered adequate for medical equipment used on smallpox patients, according to Guide F of the CDC Smallpox Response Plan (CDC 2002: Smallpox response plan and guidelines, Rutala 1996).

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Laundry and Waste

Guide F of the CDC Smallpox Response Plan also outlines the following recommendations for management of textiles and fabrics:

  • Items should be handled with a minimum amount of agitation to avoid contamination of air, surfaces, and persons.
  • Textiles and fabrics (including clothing) should be bagged at the point of use in accordance with Occupational Safety and Health Administration (OSHA) regulations. Laundry should be labeled to indicate that laundry staff should wear appropriate personal protective equipment (as specified by OSHA rules on exposure to bloodborne pathogens).
  • Laundry can be washed using routine protocols for healthcare facilities (ie, hot water [71ºC or 160ºF] with detergent, bleach, and hot air drying)

The Working Group on Civilian Biodefense recommends that bedding and clothing of smallpox patients should be autoclaved or laundered in hot water to which bleach has been added (Henderson 1999: Smallpox as a biological weapon: medical and public health management).

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Decontamination of Air Space

Laboratory dispersion studies involving vaccinia virus (as a surrogate for variola virus) indicate that infectious virions are rapidly inactivated in the environment (CDC 2002: Smallpox response plan and guidelines). In one study, only 10% to 30% of viable variola viruses were recovered from controlled aerosols after 1 hour (Mayhew 1970). Therefore, available evidence does not support air space decontamination of rooms, facilities, or vehicles (eg, fumigation). Standard terminal cleaning practices are considered adequate for rooms that have housed smallpox patients.

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Autopsy Practices

  • CDC guidelines indicate that Standard Precautions should be used for postmortem care. These include using a surgical scrub suit, surgical cap, impervious gown or apron with full sleeve coverage, a form of eye protection (eg, goggles or face shield), shoe covers, and double surgical gloves with an interposed layer of cut-proof synthetic mesh (CDC 2004: Medical examiners, coroners, and biologic terrorism: a guidebook for surveillance and case management).
  • In addition, autopsy personnel should wear N95 respirators during all autopsies, regardless of suspected or known pathogens. Powered air-purifying respirators equipped with N95 or HEPA filters should be considered.

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  • Contact with corpses should be limited to trained personnel, and routine precautions should be implemented when transporting corpses.
  • Bodies contaminated with smallpox should be cremated without embalming. If cremation is not possible, bodies should be "properly secured in a sealed container (eg, a Ziegler case or other hermetically sealed casket) to reduce the potential risk of pathogen transmission" (CDC 2004: Medical examiners, coroners, and biologic terrorism: a guidebook for surveillance and case management).
  • An example of the type of system that can be used to seal remains prior to placing them in a casket for burial is the Bioseal system, produced by Barrier Products (Barrier Products, LLC). This system uses a polyaluminum foil–extruded laminate material that, when used with a heat sealer, will provide Level 1 containment for all gases, fluids, vapors, and odors associated with the transport and storage of human and animal remains.
  • According to the WHO, "Cadavers should be cremated, in a properly designed facility, whenever possible and all persons coming in contact with them should be vaccinated or at least placed on daily fever watch. Body bags treated with hypochlorite bleach can also be used" (WHO: Smallpox).

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Public Health Reporting and Case Definitions

Clinical Case Definition
Laboratory Criteria for Confirmation
Case Classifications

A single case of smallpox is considered a public health emergency; therefore, any patients with a likely diagnosis of smallpox should be reported immediately to the state or local health department, according to disease reporting rules. Most states now have surveillance requirements and other components of a surveillance system in place (CDC 2006: Public health surveillance for smallpox—United States, 2003-2005).

Case definitions are as follows (CDC 2002: Smallpox response plan and guidelines).

Clinical Case Definition

  • An illness with acute-onset fever of 101°F (38.3°C) or more followed by a rash characterized by vesicles or firm pustules in the same stage of development without other apparent cause.

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Laboratory Criteria for Confirmation

  • Isolation of smallpox (variola) virus from a clinical specimen (LRN national laboratory) with variola PCR confirmation or
  • PCR identification of variola DNA in a clinical specimen
  • In the absence of smallpox cases (pre-event), the positive predictive value of smallpox-specific laboratory tests is low. Such testing should be reserved for cases that meet the CDC case definition for high risk of smallpox.

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Case Classifications

  • Confirmed: A case of smallpox that is laboratory-confirmed, or a case that meets the clinical case definition that is epidemiologically linked to a laboratory-confirmed case
  • Probable: A case that meets the clinical case definition, or a case that does not meet the clinical case definition but is clinically consistent with smallpox and has an epidemiologic link to a confirmed case of smallpox; examples of clinical presentations of smallpox that would not meet the ordinary type (pre-event) clinical case definition are: (a) hemorrhagic type, (b) flat-type, and (c) variola sine eruptione
  • Suspect: A case with a febrile rash illness with fever preceding development of rash by 1 to 4 days

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Resources & Literature

Recent Literature

DiEuliis D, Berger K, Gronvall G. Biosecurity implications for the synthesis of horsepox, an orthopoxvirus. Health Secur 2017 (published online Nov 1)

Esparza J, Nitsche A, Damaso C. Beyond the myths: novel findings for old paradigms in the history of the smallpox vaccine. PLoS Pathog 2018 Jul 26;14(7):e1007082

Esparza J, Schrick L, Damaso CR, et al. Equination (inoculation of horsepox): an early alternative to vaccination (inoculation of cowpox) and the potential role of horsepox virus in the origin of the smallpox vaccine. Vaccine 2017 (published online Nov 11)

Hodo CL, Mauldin MR, Light JE, et al. Novel poxvirus in proliferative lesions of wild rodents in east central Texas, USA. Emerg Infect Dis 2018 (published online May 17)

Lima MT, Oliveira GP, Assis FL, et al. Ocular vaccinia infection in dairy worker, Brazil. (Letter) Emerg Infect Dis 2017 (published online Dec 1)

MacIntyre CR, Costantino V, Chen X, et al. Influence of population immunosuppression and past vaccination on smallpox reemergence. Emerg Infect Dis 2018 (published online Feb 26)

Noyce RS, Lederman S, Evans DH. Construction of an infectious horsepox virus vaccine from chemically synthesized DNA fragments. PLoS One 2018 Jan 19;13(1):e0188453

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