IDSA | Pandemic Influenza

Last updated May 29, 2008

Agent
Laboratory Testing for Influenza
General Considerations
Historical Perspective
Pandemics of the 20th Century
Lessons From Past Pandemics
The Pandemic Severity Index
The Current H5N1 Threat
Vaccine Development
Use of Antiviral Agents
Community Mitigation Strategies
Pandemic Preparedness Planning
Hospital Pandemic Preparedness Planning
Infection Control Considerations
References

Note: Information on avian influenza is available in the overviews "Avian Influenza (Bird Flu): Implications for Human Disease" and "Avian Influenza (Bird Flu): Agricultural and Wildlife Considerations" in the Avian Flu section of this site.

Agent

All past influenza pandemics in humans have been caused by influenza A viruses. General information about influenza A viruses (not specific to pandemic strains) is presented in the bullet points below.

Family: Orthomyxoviridae

Enveloped virions are 80 to 120 nm in diameter, are 200 to 300 nm long, and may be filamentous.
They consist of spike-shaped surface proteins, a partially host-derived lipid-rich envelope, and matrix (M) proteins surrounding a helical segmented nucleocapsid (6 to 8 segments).
The family contains five genera, classified by variations in nucleoprotein (NP and M) antigens: influenza A, influenza B, influenza C, thogotovirus, and isavirus.

Genus: Influenzavirus A

The genus consists of a single species: influenza A virus.
Influenza A viruses are a major cause of influenza in humans.
The multipartite genome is encapsidated, with each segment in a separate nucleocapsid. Eight different segments of negative-sense single-stranded RNA are present; this allows for genetic reassortment in single cells infected with more than one virus and may result in multiple strains that are different from the initial ones (see References: Voyles 2002).
The genome consists of 10 genes encoding for different proteins (eight structural proteins and two nonstructural proteins). These include the following: three transcriptases (PB2, PB1, and PA), two surface glycoproteins (hemagglutinin [HA] and neuraminidase [NA]), two matrix proteins (M1 and M2), one nucleocapsid protein (NP), and two nonstructural proteins (NS1 and NS2).
The virus envelope glycoproteins (HA and NA) are distributed evenly over the virion surface, forming characteristic spike-shaped structures. Antigenic variation in these proteins is used as part of the influenza A virus subtype definition (but not used for influenza B or C viruses).

Influenza A virus subtypes:

There are 16 different HA antigens (H1 to H16) and nine different NA antigens (N1 to N9) for influenza A. Until recently, 15 HA types had been recognized, but a new type (H16) was isolated from black-headed gulls caught in Sweden and the Netherlands in 1999 and reported in the literature in 2005 (see References: Fouchier 2005).
Human disease historically has been caused by three subtypes of HA (H1, H2, and H3) and two subtypes of NA (N1 and N2).
More recently, human disease has been recognized to be caused by additional HA subtypes, including H5, H7, and H9 (all from avian origin).
All known subtypes of influenza A can be found in birds, and feral aquatic birds are the major reservoir for influenza A viruses. Feral birds generally do not develop severe disease from influenza; however, domestic chickens and turkeys are susceptible to severe and potentially fatal influenza.
Certain mammals also are susceptible to influenza. Influenza A viruses have traditionally been known to cause disease in horses, pigs, whales, and seals; however, the range of several influenza A subtypes is expanding to further mammalian species. H5N1 influenza A recently has been shown to infect cats, leopards, and tigers (see References: Keawcharoen 2004; Webster 2006). Cases of canine influenza have been recognized in the United States and are being caused by H3N8 influenza A, a subtype traditionally found in horses (see References: Crawford 2005).

Influenza A virus subtype strains

Antigenic strain nomenclature is based on: (1) host of origin (if other than human), (2) geographic origin, (3) strain number, (4) year of isolation, and (5) HA and NA type. (Examples are: A/Hong Kong/03/68[H3N2], A/swine/Iowa/15/30[H1N1].)
H5N1 strains have been differentiated into genetic clades, with nonoverlapping case distributions. All human H5N1 strains are grouped in clade 1 and subclades 1 through 3 of clade 2 (see References: WHO 2007: Antigenic and genetic characteristics of H5N1 viruses and candidate H5N1 vaccine viruses developed for potential use as pre-pandemic vaccines).

Classification of influenza A strains by pandemic potential

Strains from past pandemics: "Noncontemporary" strains are those from previous pandemics that pose some degree of risk to the public owing to decreased immunity in the current population. The term is currently used to describe strains from the Asian flu (H2N2).
Nonpandemic strains: These include strains that have recently circulated or are currently circulating in the human population (ie, those belonging to H1N1, H3N2, and H1N2 subtypes).
Potential pandemic strains: Potential pandemic strains must have the following features: (1) an antigenic makeup to which the population is immunologically naive, (2) ability to replicate in humans, and (3) capability to transmit efficiently from human to human. Because of homosubtypic immunity (see below), new pandemic strains are most likely to be of subtypes not previously recognized in human populations. Currently, strains of H5 and H7 subtypes are of greatest concern.
Animal pandemic strains (including avian influenza strains): Animal strains such as H5N1 avian influenza are not considered human pandemic strains unless the above criteria are met, but they have significant potential to evolve into new human pandemic strains through the process of genetic reassortment (see below) or through gradual adaptation to the human host. Most avian strains are not of concern as potential pandemic strains.

Avian influenza

The term "avian influenza" is used to describe influenza A subtypes that primarily affect chickens, turkeys, guinea fowls, migratory waterfowl, and other avian species.
"Avian influenza" is an ecological classification that does not correspond exactly to other classification schemes.
As with other influenza A subtypes, standard nomenclature is used to name strains (eg, A/chicken/HK/5/98 [H5N1]).
Avian influenza strains in domestic chickens and turkeys are classified according to disease severity, with two recognized forms: highly pathogenic avian influenza (HPAI), also known as fowl plague, and low-pathogenic avian influenza (LPAI). Avian influenza viruses that cause HPAI are highly virulent, and mortality rates in infected flocks often approach 100%. LPAI viruses are generally of lower virulence, but these viruses can serve as progenitors to HPAI viruses. The current strain of H5N1 responsible for die-offs of domestic birds in Asia is an HPAI strain; other strains of H5N1 occurring elsewhere in the world are less virulent and, therefore, are classified as LPAI strains. All HPAI strains identified to date have involved H5 and H7 subtypes.
Human infections have been associated with both HPAI and LPAI strains (see References: HHS 2005: Pandemic influenza plan).
Evidence that HPAI strains arise from LPAI strains has led the World Organization for Animal Health (OIE) to classify all H5 or H7 strains as notifiable (see References: Alexander 2003, Capua 2004, OIE 2005).
In the United States, currently only HPAI avian strains and reconstructed 1918 H1N1 strains are regulated as select agents (see Biosafety and Biosecurity, below).
The 1918 influenza pandemic strain (H1N1) appears to be of avian origin (see References: CDC: CDC Resources for Pandemic Flu).

Environmental survival of avian influenza viruses

Viruses remain infectious after 24 to 48 hours on nonporous environmental surfaces and less than 12 hours on porous surfaces (see References: Bean 1982). (Note: The importance of fomites in disease transmission has not been determined.)
Influenza A viruses can persist for extended periods in water (see References: WHO: Review of latest available evidence on risks to human health through potential transmission of avian influenza [H5N1] through water and sewage). One study of subtype H3N6 found that virus resuspended in Mississippi River water was detected for up to 32 days at 4°C and was undetectable after 4 days at 22°C (see References: Webster 1978). Another study found that several avian influenza viruses persisted in distilled water for 207 days at 17°C and 102 days at 28°C (see References: Stallknecht 1990).
Influenza A viruses can be preserved in lake ice and then released when the ice thaws the following spring or, in the case of arctic ice, up to years later. This may lead to temporal gene flow between viruses entrapped during one year and those shed by migrating birds in following years (see References: Zhang 2006).
Recent data from studies of H5N1 in domestic ducks have shown that H5N1 can survive in the environment for 6 days at 37ºC (see References: WHO 2004: Laboratory study of H5N1 viruses in domestic ducks).
Inactivation of the virus occurs under the following conditions (see References: OIE 2002; PHS):

Temperatures of 56°C for 3 hours or 60°C or more for 30 minutes
Acidic conditions
Presence of oxidizing agents such as sodium dodecyl sulfate, lipid solvents, and B-propiolactone
Exposure to disinfectants: formalin, iodine compounds

Laboratory Testing for Influenza

The following statements regarding laboratory testing apply to influenza viruses in general, not just to influenza testing in a pandemic setting. During a pandemic, recommendations for laboratory testing may change, depending on a number of factors, including availability of testing reagents, laboratory staffing, and surge capacity.

General Considerations

Tests for influenza virus include viral culture, polymerase chain reaction (PCR), rapid antigen testing, and immunofluorescence (IFA). Serologic tests are used to retrospectively diagnose infection.
During a pandemic, recommendations for laboratory testing may be unique and depend on factors such as: (1) availability of reagents and laboratory surge capacity, (2) presence or absence of other influenza strains in the community, (3) level of influenza activity in the community, and (4) treatment considerations.
The sensitivity and specificity of laboratory tests appear to vary with the involved strain, which has implications for emerging variants (see References: Weinberg 2005).
Laboratory tests are required for specific identification of pandemic strains. The most likely ways that a pandemic strain would be detected initially are:

Outbreak investigations or investigation of unexplained death in a previously healthy individual
Influenza surveillance with laboratory testing and characterization of unusual strains
Investigation of unusual laboratory findings
Testing of persons with influenza-like symptoms who meet certain exposure criteria

State and local health departments should be prepared to process or test for the following (if they have the capability, as described below) (see References: HHS 2005: Pandemic influenza plan).

Avian influenza A (H5N1) and other avian influenza viruses
Other animal influenza viruses
New or re-emergent human influenza viruses (such as H2 strains)

Testing during a pandemic (see References: HHS 2005: Pandemic influenza plan):

The Centers for Disease Control and Prevention (CDC) will update protocols and distribute reagents as necessary.
The need for confirmatory testing will diminish as the pandemic progresses. Some level of continued monitoring will be necessary to monitor changes in antigenicity and antiviral susceptibility. The CDC will provide appropriate guidance in such situations.

Reporting and referral (see References: HHS 2005: Pandemic influenza plan)

Clinical laboratories should contact their state or local health departments if they receive specimens from patients with possible novel influenza suspected on the basis of clinical and epidemiologic criteria.
Public health laboratories should send specimens to the CDC if the patient meets clinical and epidemiologic criteria and (1) tests positive for influenza A by reverse transcriptase PCR (RT-PCR) or rapid testing or (2) tests negative for influenza A by rapid testing and RT-PCR is not available. Laboratories without capacity for testing avian strains by indirect IFA or RT-PCR should send untypable influenza isolates to the CDC.
Any unusual subtype should be reported to the CDC through its emergency response hotline (770-488-7100).

Laboratory-based influenza surveillance networks

The World Health Organization (WHO) Global Influenza Surveillance Network (see References)
The CDC National Respiratory and Enteric Virus Surveillance System (NREVSS) (see References)
State or local health department surveillance networks

Specimen Collection

The following information is taken from a field operations guide for H5N1 influenza that was released by the WHO in early November 2006 (see References: WHO 2006: Collecting, preserving, and shipping specimens for the diagnosis of avian influenza A [H5N1] virus infection). Even though the guide is specific to H5N1 influenza, the information provided would likely be applicable to other pandemic strains. Information also was taken from the Department of Health and Human Services (HHS) Pandemic Influenza Plan where noted (see References: HHS 2005: Pandemic influenza plan [Part 2, Supplement 2]).

Specimens to collect from suspect cases

Upper respiratory tract

Posterior-pharyngeal (throat) swabs (provide the highest yield)
Nasal swabs with nasal secretions (from the anterior turbinate areas) or nasopharyngeal aspirates or swabs (these specimens are more appropriate for seasonal influenza, and the yield may be lower for avian influenza)

Lower respiratory tract

A tracheal aspirate or bronchoalveolar lavage specimen (if the patient is intubated)

Blood

Serum (acute and convalescent if possible)

Secondary specimens (to be collected as indicated early in a pandemic or under specific circumstances)

Plasma in EDTA (for detection of viral RNA)
Rectal swab (for patients with diarrhea, if the strain produces diarrhea)
Spinal fluid (if the strain produces meningitis and a spinal tap is performed for diagnostic purposes)
Pleural tap fluid (referred to in the HHS plan)
Autopsy specimens (referred to in the HHS plan)

When to collect specimens from suspect cases

Ideally, a throat swab should be taken within 3 days after illness onset; if initial specimens are negative, but if a high index of suspicion remains, testing should be repeated as soon as possible. (According to the HHS plan, specimens optimally should be collected within 4 days after illness onset.)
Virus may be detected in tracheal aspirates from onset of lower respiratory symptoms until the second or third week of illness.
An acute phase serum sample should be taken 7 days or less after symptom onset and a convalescent sample should be taken 3 to 4 weeks following illness onset.
Single serum samples should be collected 14 days or later after symptom onset.
Serum or plasma for detecting viral RNA should be obtained during the first 7 to 9 days after symptom onset.
Ideally specimens should be collected before antiviral therapy, but treatment should not be delayed to take specimens.
Specimens should be collected from deceased patients as soon as possible after death.

Specimen collection and transport

Detailed methods for specimen collection and transport are provided in the WHO field guide (see References: WHO 2006: Collecting, preserving, and shipping specimens for the diagnosis of avian influenza A (H5N1) virus infection).
Infection control precautions should be consistently observed during specimen collection.
Only sterile dacron or rayon swabs with plastic shafts should be used. Calcium alginate or cotton swabs or swabs with wooden sticks should not be used (or used only when appropriate swabs are not available).
Viral transport media (VTM) should be used for nasopharyngeal and oropharyngeal swabs, and, according to the HHS plan, specimens should be maintained at refrigerator temperature (4ºC to 8^oC) until testing is performed. Freezing at –70ºC is best for maintaining viability during extended storage.
According to the HHS plan, with regard to autopsy specimens, large airways produce the highest yield for immunohistochemistry (IHC) tests. Eight blocks or fixed-tissue specimens from each of the following sites should be obtained. Fixed tissue should be transported at room temperature (not frozen); fresh unfixed tissue should be frozen.

Central (hilar) lung with segmental bronchi
Right and left primary bronchi
Trachea (proximal and distal)
Representative pulmonary parenchyma from right and left lung

Specimen collection procedures for animals have been described by the WHO (see References: WHO 2002: Manual on animal influenza diagnosis and surveillance).

Biosafety and Biosecurity

Biosafety

Updated safety rules and recommendations for influenza virus have been included in the fifth edition of Biosafety in Microbiological and Biomedical Laboratories (BMBL) (see References: HHS 2007: Biosafety in Microbiological and Biomedical Laboratories [BMBL] 5th Edition). Current recommendations for interpandemic and pandemic alert periods include:

Biosafety level 2 (BSL-2) facilities, practices, and procedures are recommended for diagnostic, research, and production activities using contemporary, circulating human influenza strains (eg, H1/H3/B), LPAI strains (eg, H1-4, H6, H8-16) and equine and swine influenza viruses.

Animal biosafety level 2 (ABSL-2) facilities are appropriate for work with these viruses in animal models.
Use of avian and swine influenza viruses requires a permit from the Animal and Plant Health Inspection Service (APHIS) of the US Department of Agriculture (USDA).
Based on economic ramifications and source of the virus, LPAI H5 and H7 and swine influenza viruses may have additional APHIS permit-driven containment requirements, personnel practices, and/or restrictions.

Noncontemporary, wild-type human influenza (H2N2) strains should be handled with increased caution. Important considerations in working with these strains are the number of years since an antigenically related virus last circulated and the potential for a susceptible population.

BSL-3 and ABSL-3 practices, procedures, and facilities are recommended with rigorous adherence to additional respiratory protection and clothing change protocols.
Negative-pressure, high-efficiency particulate air (HEPA)-filtered respirators, or positive air-purifying respirators (PAPRs) are recommended for use.
Cold-adapted, live attenuated H2N2 vaccine strains may continue to be worked with at BSL-2 facilities.

Any research involving reverse genetics of the 1918 influenza strain should proceed with extreme caution. The risk to laboratory workers is unknown at the present time, but the pandemic potential is thought to be significant. Until further risk assessment data are available, the following practices and conditions are recommended for manipulation of reconstructed 1918 influenza viruses and laboratory animals infected with the viruses. These practices and procedures are considered minimum standards for work with the fully reconstructed virus.

BSL-3 and ABSL-3 practices, procedures, and facilities
Large laboratory animals, such as nonhuman primates, should be housed in primary-barrier systems in ABSL-3 facilities
Rigorous adherence to additional respiratory protection and clothing change protocols
Use of negative-pressure, HEPA-filtered respirators, or PAPRs
Use of HEPA filtration for treatment of exhaust air
Amendment of personnel practices to include personal showers prior to exiting the laboratory

Manipulating HPAI viruses in biomedical research laboratories requires similar caution, because some strains may pose increased risk to laboratory workers and have significant agricultural and economic implications.

BSL-3 and ABSL-3 practices, procedures, and facilities are recommended, along with clothing change and personal showering protocols.
Loose-housed animals infected with HPAI strains must be contained within agriculture-specific BSL-3 (BSL-3-Ag) facilities.
Negative-pressure, HEPA-filtered respirators or PAPRs are recommended for HPAI viruses with potential to infect humans.

When considering the biocontainment level and attendant practices and procedures for work with other influenza recombinant or reassortant viruses, the local Institutional Biosafety Committee should consider, but not limit consideration, to the following in the conduct of protocol-driven risk assessment.

The gene constellation used
Clear evidence of reduced virus replication in the respiratory tract of appropriate animal models, compared with the level of replication of the wild-type parent virus from which it was derived
Evidence of clonal purity and phenotypic stability
The number of years since a virus that was antigenically related to the donor of the HA and NA genes last circulated
If adequate risk assessment data are not available, a more cautious approach using elevated biocontainment levels and practices is warranted. There may be specific requirements regarding the setting of containment levels in institutions that are subject to National Institutes of Health (NIH) guidelines

Recommendations for testing of clinical specimens from patients suspected to have H5N1 influenza include (see References: CDC 2002: Update on influenza A [H5N1] and SARS: interim recommendations for enhanced U.S. surveillance, testing, and infection controls):

Culture from patients suspected of having avian influenza, other novel influenza strains, or severe acute respiratory syndrome (SARS) coronavirus should be conducted only under enhanced BSL-3 containment (also see Biosecurity below). This includes controlled access, double-door entry with changing room and shower, use of respirators, decontamination of all waste, and showering out of all personnel. These diagnostic activities must be kept separate from routine influenza diagnostic activities (eg, probable H1 or H3) to prevent recombination.
Indirect IFA of specimens requires BSL-2 containment and practices. Culture biocontainment recommendations should be implemented when IFA is used for culture identification.
Direct detection methods, including commercial antigen detection assays and RT-PCR, should be conducted under BSL-2 conditions with a class II biological safety cabinet. Serologic methods require BSL-2 containment.
If H5N1 avian influenza virus is presumptively identified by one of the above direct methods, further work should be conducted using the enhanced BSL-3 procedures described for culture.
Any new or re-emergent human influenza strain with suspected pandemic potential should be treated as described for H5N1 avian influenza.
Additional requirements and recommendations apply for laboratory work involving live animals.

Biosecurity

Strains of HPAI and the 1918 influenza virus are Select Agents requiring registration with the CDC and/or the USDA for possession, use, storage, and/or transfer (see References: HHS 2007: Biosafety in Microbiological and Biomedical Laboratories [BMBL] 5th Edition).

HPAI strains are agricultural Select Agents requiring registration of personnel and facilities with the lead agency for the institution (CDC or USDA-APHIS) (see References: USDA/APHIS 2005: Agricultural Bioterrorism Protection Act of 2002). An APHIS permit is required for working with these agents. Additional containment requirements, personnel practices, and/or restrictions may be added as conditions of the permit.
Both registered and exempt laboratories that identify a Select Agent contained in a specimen presented for diagnosis, verification, or proficiency testing must secure the agent against theft, loss, or release until transfer or destruction. Unregistered laboratories must transfer or destroy select agents within 7 days of identification. Any theft, loss, or release of the agent must be reported to the select agent authority (see References: USDA/APHIS 2005: Questions and answers).

Direct Detection Methods

Direct detection methods do not result in production of an isolate and would be inadequate for surveillance or definitive characterization of pandemic strains. Nevertheless, owing to their relatively rapid turnaround time, safety, and stability, direct detection methods play an important role in pandemic influenza preparedness.

RT-PCR assays

RT-PCR assays use conserved targets such as the M protein for genus-level identification. HA and NA targets are used for specific identification of avian subtypes. PCR generally is not used for strain-level identification, which is based on serologic markers.
The sensitivity of RT-PCR has been reported to be in the range of 90% to 100% when compared with cell culture. However, several researchers have reported significantly higher numbers of total positive specimens with RT-PCR, possibly reflecting its ability to detect nonviable virions (see References: Coiras 2003, Hayden 2002, Herrmann 2001, Pachucki 2004, Wallace 1999).
In February 2006, the Food and Drug Administration (FDA) announced clearance of an Influenza A/H5 (Asian Lineage) Virus Real-Time Reverse Transcription–Polymerase Chain Reaction (RT-PCR) Primer and Probe Set and inactivated virus as a source of positive RNA control for the in vitro detection of HPAI H5 virus (Asian lineage) (see References: CDC 2006: New laboratory assay for diagnostic testing of avian influenza A/H5 [Asian lineage]). These reagents and assay protocols have been distributed by the CDC to state and city LRN (Laboratory Response Network) laboratories. Testing with the new assay is limited to LRN-designated laboratories.
While culture of specimens from possible avian influenza (H5N1) cases is not recommended without strict containment and specific registration (described above), RT-PCR can be conducted using BSL-2 facilities and practices (see References: HHS 2005: Pandemic influenza plan).
Common PCR targets include the M protein (for genus-level identification) and HA and NA (for subtype-level identification). PCR generally is not used for strain-level identification, which is based on serologic markers.
The likelihood that an RT-PCR assay will detect new pandemic strains increases when more conserved target sequences are used.
As with other PCR-based assays, efforts should be made to minimize and detect amplicon contamination.
Samples positive by RT-PCR for a novel influenza subtype should be forwarded to a public health laboratory (if testing was conducted at a private laboratory) or to the CDC for confirmation (see References: HHS 2005: Pandemic influenza plan).
The development of portable real-time platforms has made possible the use of PCR assays in the field (see References: Perdue 2003).

Immunofluorescence

IFA methods may be used to identify influenza to the species level (influenza A or B) or specific H subtypes (including H5) directly from specimens or cell culture. The CDC distributes IFA typing and subtyping reagents to WHO-collaborating laboratories, including many health department laboratories. If HPAI strains are suspected, enhanced BSL-3 containment should be used (see References: WHO 2004: Recommended laboratory tests to identify avian influenza A virus in specimens from humans; FDA: Cautions in using rapid tests for detecting influenza A viruses; HHS 2005: Pandemic influenza plan).
Direct immunofluorescence (DFA) methods are faster and less labor intensive than IFA but are less sensitive and are currently only available for genus-specific detection (see other rapid direct tests in the next bullet point).

Molecular microarray tests using flow-through chip technology

A molecular microarray for influenza typing and subtyping using a "flow-thru chip platform" was initially described in 2004 (see References: Kessler 2004).
Two reports released in August 2006 involved a study of the FluChip-55 diagnostic microarray and showed that the test could be a valuable tool in identifying influenza viruses (see References: Mehlmann 2006, Townsend 2006). The FluChip used in the study contained 55 sequences of RNA representing a variety of type A and type B flu viruses, including H3N2, H1N1, and H5N1. Combined results after two rounds of testing showed that the FluChip allowed users to obtain correct information about both type and subtype from 72% of 72 samples tested. Full information on type, but only partial information on subtype, was obtained for an additional 13% of the samples, while 10% of the samples could be identified by type only (no information about subtype). The entire analysis time was less than 12 hours.
Scientists recently have developed an improved microarray test referred to as the “MChip,” which has several advantages over the FluChip. While the FluChip is based on three influenza genes—HA, NA, and M—the MChip is based on only the M gene segment, which mutates much less rapidly. A recent evaluation demonstrated that the assay exhibited a clinical sensitivity of 97% and clinical specificity of 100% (see Nov 15, 2006, CIDRAP News Story).

Other rapid direct tests (see References: Call 2005; CDC 2004: Interim guidance for influenza diagnostic testing during the 2004-05 influenza season; FDA 2005: Cautions in using rapid tests for detecting influenza A viruses; HHS 2005: Pandemic influenza plan; Treanor 2005; WHO 2005: WHO checklist for influenza pandemic preparedness planning).

Rapid tests detect viral antigen (generally nucleoprotein) or enzymatic activity (NA) directly on patient specimens using a variety of platforms.
Rapid tests are designed to identify influenza A only, influenza A or B without identifying the type, or influenza A or B with type-specific identification.
Reported sensitivities range from 40% to 80%.
Sensitivity is generally greater in children than adults.
Sensitivity is greater early in the course of illness.
Rapid test predictive value and disease prevalence: The predictive value of rapid assays without confirmation by a reference test is strongly correlated with disease prevalence in the community, as is clinical diagnosis without laboratory testing. When the disease prevalence is low, the tests' positive predictive value decreases, and positive results should be confirmed by culture or RT-PCR. When influenza is known to be circulating (ie, high prevalence in the community), the negative predictive value is lower, and clinicians should consider confirming negative tests with viral culture or other tests.
Rapid test predictive value and diagnostic indications: Rapid tests increase the diagnostic predictive value when used for confirmation of influenza (when symptoms are strongly suggestive) and for ruling out influenza (when symptoms suggest illness other than influenza). When symptoms are not strongly suggestive in either direction, the utility of rapid testing becomes questionable.
While the sensitivity and specificity of rapid tests has been evaluated for circulating strains, these measures are largely unknown for detection of emerging strains (including pandemic strains) (see References: FDA 2005).
The WHO, in its Checklist for Influenza Pandemic Preparedness Planning, recommends against routine use of commercial rapid antigen detection kits and suggests they be used for outbreak investigation only when no other options exist (see References: WHO Writing Committee of WHO Consultation on Human Influenza A/H5 2005).
During a pandemic, rapid tests may be useful for distinguishing influenza from other respiratory illnesses (see References: HHS 2005: Pandemic influenza plan).

Serology

Serologic testing can be used for retrospective diagnosis of infection but is rarely useful for patient management and is not widely available. However, serology may be useful for investigation of novel viruses (see References: Hayden 2002; Treanor 2005; HHS 2005: Pandemic influenza plan).
Acute-phase sera should be collected within 1 week after illness onset, and convalescent sera should be collected 2 to 3 weeks later.
The most common serologic methods are complement fixation (CF), hemagglutination inhibition (HAI), and enzyme immunoassays (EIAs). A variety of other methods, such as neutralization, microneutralization, single radial hemolysis, radial immunodiffusion, and Western blot, have been reported (see References: Hayden 2002, Rowe 1999).
Immunoglobulin type G (IgG), IgA, and IgM antibodies appear simultaneously about 2 weeks after initial infection. Antibodies appear more quickly with subsequent infections. Tests for IgM and IgA are less useful than for IgG for routine clinical use, as most infections are reinfections (see References: Australian Government Department of Health and Ageing; Hayden 2002)
Peak antibody response occurs 4 to 7 weeks after infection.
Since most people are repeatedly exposed to influenza viruses, a fourfold rise in titer between acute and convalescent sera generally is considered necessary for confirmation of influenza infection.
While paired sera are optimal, single convalescent specimens may be useful in investigations involving novel viruses. Antibody test results have been compared with results from age-matched persons in the acute phase of illness or from non-ill controls. The geometric mean titers between the two groups to a single influenza virus type or subtype can be compared (see References: HHS 2005: Pandemic influenza plan).
HAI and EIAs measure antibody to HA. These tests are more sensitive than CF, but their increased specificity appears to limit their ability to detect new strains.
HAI titers of at least 1:40 or serum neutralizing titers of 1:8 or greater are associated with protection.
HAI titers in human avian influenza cases have been low or undetectable (see References: HHS 2005: Pandemic influenza plan).
CF measures antibody response to nucleoprotein, which is conserved among influenza A strains. This feature could be an advantage for diagnosis of infection with novel pandemic strains.
The microneutralization assay can sensitively and specifically detect H5N1 antibody in patients with H5N1 influenza. Since the test uses infectious organisms, HPAI strains should be tested under enhanced BSL-3 containment. As with other tests, paired sera are preferable to single specimens (see References: HHS 2005: Pandemic influenza plan).

Virus Isolation by Cell Culture

Virus isolation is considered the "gold standard" of influenza testing (see References: Hayden 2002, Treanor 2005).
Cell culture measures growth rather than the presence or absence of specific targets. As cell lines are designed to support the growth of a wide range of viruses, cell culture will likely allow for detection of emerging and pandemic influenza strains (see References: Australian Government Department of Health and Ageing 2000).
Isolates obtained from cell culture are required for strain characterization, which is an integral part of global influenza surveillance and monitoring activities during a pandemic (see References: HHS 2005: Pandemic influenza plan).
Cell culture is subject to certain restrictions (see Biosafety and Biosecurity above).
Specimens for culture optimally should be collected within 3 days after illness onset.
Turnaround time for the standard method is 2 to 14 days.
Culture consists of growth on a cell monolayer, detection of viral growth, and specific identification.
Virus detection and identification methods for standard culture include the following:

Cell lines include Madin-Darby canine kidney (MDCK), primary rhesus monkey kidney (PRMK), or cynomolgus monkey kidney. Other cell lines, such as Vero, mink lung, and MRC-5, also support growth of influenza virus if trypsin is incorporated into serum-free medium.
Cytopathic effect (CPE) is not a consistent feature of influenza A virus. If present, CPE is nonspecific, including vacuolization or cell degeneration.
Assays for haemadsorption (HAd) (ie, influenza-infected cells bind red blood cells [RBCs]) are performed blindly, typically at 7 and 14 days or on cells exhibiting CPE. Other viruses, such as parainfluenza virus and mumps virus, may also cause HAd. The lack of HAd specificity may be an advantage in detecting new or pandemic strains.
HAI is used to identify the viral subtype. Cell supernatant is mixed with RBCs; identification is by quantitative inhibition of agglutination using subtype-specific antisera. Homologous strains yield high HAI titers. New pandemic strains would likely be HAd-positive with or without CPE, with low or negative titers to group-specific antisera.
Identification of infected cells is by direct or indirect immunofluorescence (eg, DFA, IFA, EIA) or PCR-based methods. Assays with more conserved, less specific targets are more likely to detect newly emerged strains.
The time to detection in culture, as measured in one study conducted during two influenza seasons, ranged from 5 days (>90% of positive specimens) to 7 days (100% of positive specimens) (see References: Newton 2002).
A golden rule of laboratory testing is to never process clinical specimens from humans and swine (and presumably birds) in the same laboratory (see References: WHO 2004: Recommended laboratory tests to identify influenza A/H5 in specimens from patients with influenza-like illness).

Shell vial assay (rapid culture), when combined with a rapid detection/identification method, offers a sensitive and rapid diagnostic alternative to standard culture. This method does not result in an adequate viral titer or volume for further characterization and would thus not be appropriate for pandemic influenza surveillance without subculture.

Susceptibility Testing

Susceptibility testing generally is conducted at specialized laboratories as part of surveillance or research and is considered an integral component of pandemic influenza response.
Plaque reduction assay (see References: Hayden 1980, McKimm-Breschkin 2003)

The traditional influenza susceptibility testing method for the M2 ion channel inhibitors (amantadine, rimantadine)
Can detect a wide range of resistance phenotypes
Limited utility for NA inhibitors

Enzyme inhibition assays (see References: McKimm-Breschkin 2003, Wetherall 2003)

Useful for assay of NA inhibitors
Chemiluminescent or fluorescent substrates

Sequence analysis (see References: McKimm-Breschkin 2003, Wetherall 2003)

Used to detect mutations in genes known to be or suspected of being responsible for resistance
NA gene sequences from strains isolated prior to introduction of the drugs can be used to evaluate current strain sequences
Mutations in the M2 can be used to detect amantadine resistance (see References: Pachucki 2004)

Researchers have recently reported a PCR assay to efficiently and accurately detect oseltamivir-sensitive and oseltamivir-resistant H5N1 strains (see References: Suwannakarn 2006). The assay is based on the fact that oseltamivir resistance is caused by a single amino acid substitution from histidine (H) to tyrosine (Y) at position 274 of the NA active site.
The Neuraminidase Inhibitor Susceptibility Network (NISN) was established to monitor susceptibility of clinical isolates to zanamivir and oseltamivir. The chemiluminescent NA enzyme assay was chosen by the NISN as the method of choice for testing NA inhibitors (see References: Wetherall 2003).

Laboratory Values That May Trigger Concern for Human Pandemic Influenza

Positive test for influenza from a patient with risk factors for avian influenza
Culture: CPE positive or negative; HAd positive; HAI titer low or negative and no other hemagglutinating viruses identified
RT-PCR positive for H5 or H7
RT-PCR positive for influenza A from a conserved target, such as matrix protein, and negative for H1-H3
A four-fold rise in H5-specific antibody titer (acute and convalescent serum samples)

General Considerations

Cross-Immunity

In general, the degree of immunity induced by one strain of influenza virus to a second challenge with another influenza virus is related to the taxonomic distance between the two strains (see References: Epstein 2003). Several terms that characterize the type of immunity are identified below.

Heterologous immunity: Immunization with one type of influenza virus (eg, A, B, or C) does not offer protection from challenge with a different type.
Heterosubtypic immunity (also referred to as "heterotypic immunity"): Immunization with one influenza A virus subtype (eg, H1N1) may offer some protection from challenge with a second influenza A subtype (eg, H5N2). The degree of protection, or lack of protection, is important to the discussion of pandemic influenza and vaccine development.
Homosubtypic immunity: Immunization with one strain within a subtype (eg, A/Hong Kong/03/68[H3N2]) will frequently offer some protection against challenge with a second strain within the same subtype (eg, A/Fujian/447/2003[H3N2]).
Homologous immunity: Immunization with one strain of influenza A virus (eg, A/Fujian/447/2003[H3N2]) offers protection from a second challenge with the same strain.

Antigenic Drift vs Antigenic Shift

"Antigenic drift" refers to the process of small genetic changes that influenza viruses continuously undergo from year to year, which necessitates the development of new vaccines annually. Partial immunologic cross-reactivity between new strains and those they are replacing (ie, homosubtypic immunity) limits morbidity, mortality, and spread in the population.
"Antigenic shift" refers to substantial genetic changes caused by the process of genetic reassortment. Relatively few lineages of influenza A are circulating among humans at any one time, which reduces the likelihood of significant genetic reassortments. However, antigenic shift can occur between human and animal strains, which is what happened with the pandemic strains of 1957 and 1968. It is important to note that not all pandemic strains arise from genetic reassortment. For example, the 1918 pandemic strain apparently did not originate through a reassortment event; rather, it is likely that an avian strain initially infected humans and then adapted gradually to the human population over time to become a pandemic strain (see References: Taubenberger 2005).

Features of Pandemic Strains

Pandemics occur when a novel influenza strain emerges that has the following features:

Highly pathogenic for humans
Easily transmitted between humans
Genetically unique (ie, lack of preexisting immunity in the human population)

Pandemic Phases

In reviewing the public health implications of a pandemic, it is useful to understand the various phases that a pandemic will likely go through. These are outlined in the following table. (Note: In 1999, the WHO developed a set of pandemic phases; these were revised in the new WHO Global Influenza Preparedness Plan that was released in April 2005. The phases identified below are from the 2005 Plan [see References: WHO: WHO global influenza preparedness plan 2005].) The current pandemic phase for H5N1 is phase 3.

WHO Pandemic Phases
Phase	Characteristics of Phase	Rationale
Phase 1	No new influenza virus subtypes have been detected in humans. An influenza virus subtype that has caused human infection may be present in animals. If present in animals, the risk of human infection or disease is considered to be low.	It is likely that influenza subtypes that have caused human infection and/or disease will always be present in wild birds or other animal species. Lack of recognized animal or human infections does not mean that no action is needed. Preparedness requires planning and action in advance.
Phase 2	No new influenza virus subtypes have been detected in humans. However, a circulating animal influenza virus subtype poses a substantial risk of human disease.	The presence of animal infection caused by a virus of known human pathogenicity may pose a substantial risk to human health and justify public health measures to protect persons at risk.
Pandemic Alert Period
Phase 3	Human infection(s) with a new subtype, but no human-to-human spread, or at most rare instances of spread to a close contact.	The occurrence of cases of human disease increases the chance that the virus may adapt or reassort to become transmissible from human to human, especially if coinciding with a seasonal outbreak of influenza. Measures are needed to detect and prevent spread of disease. Rare instances of transmission to a close contact, for example, in a household or healthcare setting, may occur but do not alter the main attribute of this phase (ie, that the virus is essentially not transmissible from human to human).
Phase 4	Small cluster(s) with limited human-to-human transmission but spread is highly localized, suggesting that the virus is not well adapted to humans.	Virus has increased human-to-human transmissibility but is not well adapted to humans and remains highly localized, so that its spread may possibly be delayed or contained.
Phase 5	Larger cluster(s) but human-to-human spread is still localized, suggesting that the virus is becoming increasingly better adapted to humans but may not yet be fully transmissible (substantial pandemic risk).	Virus is more adapted to humans and therefore more easily transmissible among humans. It has spread in larger clusters, but spread is localized. This is likely to be the last chance for massive coordinated global intervention, targeted to one or more foci, to delay or contain spread. In view of possible delays in documenting spread of infection during pandemic Phase 4, it is anticipated that there would be a low threshold for progressing to Phase 5.
Pandemic Period
Phase 6	Increased and sustained transmission among general population.	Major change in global surveillance and response strategy, since pandemic risk is imminent for all countries. The national response is determined primarily by the disease impact within the country.
From WHO: WHO global influenza preparedness plan 2005 (see References).

Historical Perspective

Earliest reports of influenza epidemics date back to 412 BC and were recorded by Hippocrates. A number of epidemics that likely were influenza were described in the Middle Ages, and one that was probably a true pandemic took place in 1510 (see References: Beveridge 1978). Other key historical facts include the following:

One of the earliest recorded pandemics occurred in 1580. Like the 1918 pandemic, this one was particularly severe. It started in Asia and spread to Africa, Europe, and the Americas. In 6 weeks it afflicted all of Europe. Death rates were high; 9,000 of 80,000 people died in Rome, and some Spanish cities were described as "nearly entirely depopulated" by the disease (see References: Beveridge 1978, Patterson 1986).
Ten pandemics have been recorded in the past 300 years (see References: Osterholm 2007: The fog of pandemic preparedness). The time between starting points of these pandemics has ranged from 10 to 49 years, with an average of 24 years.
During the 17th century, localized epidemics were reported, and in the 18th century at least two pandemics occurred (1732-33, and 1781-82).
Five influenza pandemics occurred during the 19th century (1800-02, 1830-33, 1847-48, 1857-58, and 1889-90) (see References: Osterholm 2007: The fog of pandemic preparedness). The 1889 pandemic, known as the Russian Flu, began in Russia and spread rapidly throughout Europe. It reached North America in December 1889 and spread to Latin America and Asia in February 1890. About 1 million people died in this pandemic.

Global influenza surveillance was established in 1947 by the WHO to better understand the epidemiology of influenza and to obtain isolates in a systematic fashion for annual vaccine development (see References: Hampson 1997).

Pandemics of the 20th Century

Three pandemics occurred during the 20th century, caused by an H1, an H2, and an H3 strain. These are outlined in the table below and then briefly summarized. Currently, H1 and H3 influenza strains are circulating in the human population. Scientists have raised concern about the possibility of H2N2 reemerging (also referred to as recycling) in humans, particularly through accidental release of a laboratory strain (see References: Dowdle 2006).

Influenza Pandemics of the 20th Century: Impact in the United States*
Date	Strain	Estimated No. of Deaths in US	Comments
1918-19 (Spanish Flu)	H1N1	500,000	Global mortality may have been as high as 100 million. The virus likely originated in the US and then spread to Europe.
1957-58 (Asian Flu)	H2N2	60,000	The virus was first identified in China; approximately 1 million people died globally during this pandemic.
1968-69 (Hong Kong Flu)	H3N2	40,000	The death rate from this pandemic may have been lower because the strain had a shift in the hemagglutinin (H) antigen only and not in the neuraminidase (N) antigen.
*All three pandemics were characterized by a shift in age distribution of deaths to younger populations under age 65 (at least initially); shift was particularly dramatic during the 1918 pandemic (see References: HHS: Q&A; HHS: Influenza pandemics; Kilbourne 2006; Simonsen 2004; Webster 1997).

1918-19 (Spanish Flu)

This pandemic was caused by an influenza A (H1N1) strain. Worldwide, about one third of the world's population was infected and had clinically apparent illness (about 500 million people) and an estimated 50 to 100 million died (see References: Johnson 2002, Taubenberger 2006). Earlier estimates implied that the death toll was 20 to 40 million, but more recent evidence supports the higher figures. Adjusting for today's population, a similar pandemic would yield a modern death toll of 175 to 350 million.

One recent study projected 51 to 81 million deaths using 2004 population estimates; however, the authors assumed wide variability in death rates by country based on per-capita income and other factors (see Dec 22, 2006, CIDRAP News Story and see References: Murray 2006).
Another recent report suggests that if a 1918-like pandemic were to occur with increased deaths in the elderly population, over 142.2 million people would die and there would be a gross domestic product loss of US $4.4 trillion worldwide (see References: Osterholm 2007: Unprepared for a pandemic).
Some have suggested that the death toll from a similar pandemic occurring in modern times would be lower owing to improved medical care and public health infrastructure (see References: Morens 2007); however, if attack rates were high, medical and public health systems could quickly become overwhelmed.

The 1918 pandemic began with a relatively mild "herald" wave in the spring of 1918. During that time, outbreaks were reported in Europe and in the United States (particularly in military training camps for new recruits headed to the war in Europe) (see References: Reid 2001, Glezen 1996).

Many investigators believe that the strain originated in the United States (perhaps in rural Kansas) and then migrated initially to France before spreading throughout Europe (see References: Barry 2004). However, others believe that the strain may have been circulating in the Mid-Atlantic states as early as February of 1918 (see References: Simonsen 2004). Furthermore, an outbreak of severe respiratory disease occurred in an army camp in France in 1916-17 (see References: Oxford 2000). A significant clinical feature of this disease was cyanosis, which also was a predominant finding among those who acquired the pandemic strain of influenza. It is possible that this outbreak represented H1N1 infection and was an early precursor to the pandemic. At any rate, it is clear that the 1918-19 pandemic did not begin in Asia, although the origin of the implicated H1N1 strain still remains a mystery.
This first wave was followed by two additional waves in the fall and winter of 1918-19 that were much more severe (see References: Taubenberger 2006). The second, highly virulent, wave spread rapidly around the world in the fall of 1918; it took only 2 months for the pandemic to circle the globe at that time.
Recorded case-fatality rates (CFRs) varied around the globe. In the US military, death rates ranged from 5% to 10% (see References: Barry 2004). Higher rates were reported in some areas.
Additional waves that were not as severe occurred in 1919 and 1920.

An unusual feature of the pandemic was the age-related mortality; the pandemic strain killed a disproportionate number of healthy young adults. This led to the observation of a "W" shaped age-related mortality curve in the United States, with high rates of mortality among very young children, persons 15 to 45 years of age, and the elderly (see References: Reid 2001; Glezen 1996; Morens 2007). Usually the curve associated with influenza mortality follows a "U" shape, with excess deaths occurring only among the very young and the elderly. One striking feature of the pandemic was its impact on pregnant women; a summary of 13 studies involving pregnant women demonstrated that CFRs ranged from 23% to 71% (see References: Barry 2004).

The excess influenza deaths appear to have involved two overlapping clinical-pathologic syndromes (see References: Morens 2007). One pattern was aggressive bronchopneumonia, most likely caused by a secondary bacterial pneumonia. The second pattern was a rapidly evolving severe acute respiratory distress-like syndrome (ARDS).

In October 2005, the CDC reported that scientists had reconstructed the 1918 pandemic H1N1 strain and tested it in mice (see References: Tumpey 2005). They found that mice infected with the 1918 strain died in as little as 3 days, and mice that survived as long as 4 days had 39,000 times as many virus particles in their lungs as did mice infected with a control influenza virus, a Texas strain of H1N1 from 1991. All the mice infected with the 1918 virus died, while those exposed to the Texas strain survived. Further, the 1918 virus was at least 100 times as lethal as an engineered virus that contained five 1918 genes and three genes from the Texas H1N1 strain. The researchers found that the mice had severe inflammation in their lungs and bronchial passages, findings very similar to those in people who died of the 1918 virus.

Earlier studies in mice using genetically engineered influenza strains similar to the H1N1 1918 pandemic strain suggest that macrophage activation with high levels of cytokine production may have been a key factor in lung damage caused by the pandemic strain (see References: Kobasa 2004). Investigators have postulated that an overly robust immune response inducing a "cytokine storm" may have contributed to the high CFRs seen in younger populations during the 1918 pandemic. Another study recently found that cynomolgus macaques had an atypical host response to infection with the 1918 virus (characterized by dysregulation of the antiviral response), suggesting that the 1918 virus was able to modulate the host immune response (see References: Kobasa 2007).

Recent genetic sequencing of the 1918 strain indicates that the strain was of avian origin and that the strain did not reassort with a human strain (unlike later pandemics), but rather gradually adapted to humans until it could be efficiently transmitted person to person (see References: Taubenberger 2005). Current evidence indicates that the 1918 virus was an avian-like virus derived in toto from an unknown source (see References: Taubenberger 2006). A two-amino acid change in the HA of the 1918 virus was recently shown to abolish transmission among ferrets, confirming the essential role of HA receptor specificity for the transmission of influenza viruses in mammals (see References: Tumpey 2007).

1957-58 (Asian Flu)

The Asian flu was caused by an H2N2 strain and originated in China. The virus was initially isolated in Singapore in February 1957 and in Hong Kong in April of that year. The pandemic spread to the Southern Hemisphere during the summer of 1957 and reached the United States in June 1957 (see References: Glezen 1996). The pandemic strain acquired three genes from the avian influenza gene pool in wild ducks by genetic reassortment and obtained five other genes from the then-circulating human strain.

About 69,800 people in the United States died and mortality was spread over three seasons. Overall, the highest mortality rates occurred among the elderly; however, during the initial season in 1957, nearly 40% of the influenza deaths occurred among persons less than 65 years of age (see References: Simonsen 2004). The high CFR in this age-group declined in subsequent years. Globally, approximately 1 million people died during this pandemic.

1968-69 (Hong Kong Flu)

The Hong Kong flu was caused by an H3N2 strain. The strain acquired two genes from the duck reservoir by reassortment and kept six genes from the virus circulating at the time in humans.

During the pandemic, about 33,800 people died in the United States. The death rate from this pandemic may have been lower because the strain had a shift in the HA antigen only and not in the NA antigen. Although antibodies to NA antigen do not prevent infection, they may modify the severity of disease (see References: Glezen 1996). Also, an H3 strain had apparently circulated in the United States around the turn of the century, so elderly persons may have had some protective antibody from past exposure to an H3 strain (see References: Simonsen 2004). This could have caused a lower fatality rate in the elderly.

Lessons From Past Pandemics

In a report issued in January 2005, WHO officials identified key lessons from the three pandemics of the past century (see References: WHO 2005: Avian influenza: assessing the pandemic threat). These lessons are summarized as follows.

Pandemics behave as unpredictably as the viruses that cause them. During the previous century, great variations were seen in mortality, severity of illness, and patterns of spread.
One consistent feature important for pandemic preparedness planning is the rapid surge in the number of cases and their exponential increase over a very brief time, often measured in weeks.
Apart from the inherent lethality of the virus, its capacity to cause severe disease in non-traditional age groups, namely young adults, is a major determinant of a pandemic's overall impact.
The epidemiologic potential of a virus tends to unfold in waves. Subsequent waves have tended to be more severe.
Virologic surveillance, as conducted by the WHO Laboratory Network, has performed a vital function in rapidly confirming the onset of pandemics.
Most pandemics have originated in parts of Asia where dense populations of humans live in close proximity to ducks and pigs.
Some public health interventions may have delayed the international spread of past pandemics, but could not stop them.
Delaying spread is desirable, as it can flatten the epidemiological peak, thus distributing cases over a longer period.
The impact of vaccines on a pandemic, though potentially great, remains to be demonstrated. In 1957 and 1968, vaccine manufacturers responded rapidly, but limited production capacity resulted in the arrival of inadequate quantities too late to have an impact.
Countries with domestic manufacturing capacity will be the first to receive vaccines.
The tendency of pandemics to be most severe in later waves may extend the time before large supplies of vaccine are needed to prevent severe disease in high-risk populations.
In the best-case scenario, a pandemic will cause excess mortality at the extremes of the lifespan and in persons with underlying chronic disease. Countries with good programs for yearly influenza vaccinations will have experience with the logistics of vaccinations for these populations.

The Pandemic Severity Index

In February 2007, HHS released the "pandemic severity index," or PSI, as a way to grade pandemics. The severity level is initially based on CFR, a single criterion that will likely be known even early in a pandemic when small clusters and outbreaks are occurring. The pandemic severity index levels are:

Category 1, CFR <0.1%
Category 2, CFR 0.1% to 0.5%
Category 3, CFR 0.5% to 1%
Category 4, CFR 1% to 2%
Category 5, CFR >2%

According to this index, the pandemics of 1957 and 1968 both fit into category 2, whereas the 1918 pandemic qualified as a category 5.

The Current H5N1 Threat

According to the WHO, at this time the pandemic alert level for H5N1 influenza is at Phase 3: a new viral subtype is causing disease in humans but is not yet spreading efficiently and sustainably (see References: WHO: Current WHO phase of pandemic alert).

Detailed information about H5N1 influenza in bird populations can be found in the document on this Web site, "Avian Influenza (Bird Flu): Agricultural and Wildlife Considerations" and in human populations in the document, "Avian Influenza (Bird Flu): Implications for Human Disease."

Of the avian influenza subtypes, currently the H5N1 subtype is of greatest pandemic concern for the following reasons (see References: WHO: Avian influenza fact sheet; WHO 2005: Influenza pandemic preparedness and response):

Since 2003, H5N1 viruses have spread across Asia and into Europe, the Middle East, India, and Africa, with outbreaks occurring in bird populations in more than 60 countries.
In early 2008, the United Nations Food and Agriculture Organization (FAO) reported that the greatest areas of ongoing concern are Indonesia, Bangladesh, and Egypt, where the virus has become "deeply entrenched" (see References: FAO 2008). The potential of exposure and infection of humans is likely to be ongoing in rural areas of these countries, which could enhance the likelihood that a pandemic strain will emerge (see References: Stohr 2005).
H5N1 strains cause severe disease in humans, with a high CFR (reported at over 60%).
Recent genetic sequencing performed on H5N1 viral isolates from Turkey demonstrates that the strains contain two mutations that may make the virus better adapted to humans (see References: Butler 2006). These mutations could potentially enhance transmission from birds to humans and between humans.

Since 2003, human cases of H5N1 influenza have been reported in Azerbaijan, Bangladesh, Cambodia, China, Djibouti, Egypt, Indonesia, Iraq, Lao People's Democratic Republic, Myanmar, Nigeria, Pakistan, Thailand, Turkey, and Vietnam.

The WHO has officially recognized more than 380 cases of H5N1 influenza (see References: WHO: Cumulative number of confirmed human cases of avian influenza), with an overall CFR higher than 60%.
An epidemiologic report on 340 confirmed H5N1 influenza cases published by the WHO in January 2008 demonstrated that the median age of cases was 18 years and that 90% of infections occurred in persons under 40 years of age (see References: WHO Writing Committee of the Second World Health Organization Consultation on Clinical Aspects of Human Infection with Avian Influenza A [H5N1] Virus 2008). The overall CFR was 61% and was highest among persons 10 to 19 years of age and lowest among persons 50 years of age or older. Of six infected pregnant women, four died and two had spontaneous abortions.

The high CFR suggests that the pathogenicity of H5N1 ma