Epidemiology

Last updated December 16, 2010. At the current time, this content is considered historical and will not be updated until further notice.

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Preexisiting Immunity to the pH1N1 2009 Strain
Transmission
Household Transmissibility Studies
Infectious Period and Viral Shedding
Groups at High Risk for Serious Illness and Complications

Preexisting Immunity to the pH1N1 2009 Strain

At the beginning of the pandemic, the CDC indicated that adults appeared to have some degree of preexisting immunity to the pandemic strain, especially those 60 years of age or older. The CDC postulated that some adults in this age-group may have had previous exposure (through either prior infection or remote vaccination) to an influenza A H1N1 virus genetically and antigenically more closely related to the pH1N1 2009 virus than contemporary seasonal H1N1 strains (CDC 2009: Serum cross-reactive antibody response to a novel influenza A (H1N1) virus after vaccination with seasonal influenza vaccine).

Several additional studies in Europe also suggested that the elderly had some pre-pandemic immunological protection (Ikonen 2010, Miller 2010, Rizzo 2010). For example, in a study of 1,000 elderly individuals in Finland, investigators found that 96% of persons born from 1909 to 1919 had cross-reactive antibodies against the pH1N1 2009 virus and that persons born from 1920 to 1944 had a varying prevalence of such antibodies (14% to 77%); however, most persons born after 1944 lacked cross-reactive antibodies to the pandemic strain (Ikonen 2010).

Transmission

The pH1N1 2009 virus is transmitted in ways similar to other influenza viruses, including large-particle respiratory droplet transmission (eg, when an infected person coughs or sneezes near a susceptible person). Transmission via aerosols also may be important, although animal studies using a ferret model are somewhat conflicting on this issue (Maines 2009, Munster 2009). The relative contributions of small-particle aerosols, large droplets, and fomites remain uncertain.
According to the WHO, the estimated reproduction number or R0 (which is the number of secondary cases generated by each primary case) during the pandemic ranged from 1.3 to 1.7 in most populations (Writing Committee of the WHO Consultation on Clinical Aspects of Pandemic H1N1 2009 Influenza). This number is similar to or slightly higher than estimates for seasonal influenza. Other studies have found a range in R0 from 1.4 to 2.3 (Boelle 2009, Fraser 2009, Nishiura 2010, White 2009).
For more information on transmission, see the section on Infection Control Considerations.

Household Transmissibility Studies

Several studies have examined transmissibility of pH1N1 2009 virus in households.

One study found that the transmissibility of the pH1N1 2009 influenza virus in households was lower than that seen in past pandemics (Cauchemez 2009). The proportion of household contacts in whom acute respiratory illness developed decreased with the size of the household (28% in two-member households, 9% in six-member households) (Cauchemez 2009).).
Another study found that the secondary attack rates (as confirmed on reverse-transcriptase polymerase-chain-reaction [RT-PCR] assay) among household contacts of index patients were similar for the pandemic influenza virus (8%) and seasonal influenza viruses (9%) (Cowling 2010). The patterns of viral shedding and the course of illness among index patients were also similar for the pandemic and seasonal influenza viruses.
In another study, 77 households were investigated to assess household transmission of pH1N1 2009 in San Antonio , Texas, from Apr 15 through May 8, 2009. Secondary attack rates were highest in children <5 years of age and were higher in children 5 to 18 years of age than in adults. Secondary attack rates were highest in households with 2 to 3 persons and were lowest in households with 7 to 9 persons (Morgan 2010: Household transmission).
The effect of oseltamivir treatment on the household and individual secondary attack rates was assessed in 135 households in Milwaukee (Goldstein 2010). Oseltamivir treatment on the day of onset or the following day of the index case was associated with a 42% reduction in the odds of one or more secondary infections in a household and a 50% reduction in the odds of a secondary infection in individual contacts.

Infectious Period and Viral Shedding

Several studies have examined viral shedding following infection with pH1N1 2009 virus. Examples include the following:

One prospective study showed that the proportion of infected persons still shedding replicating virus 8 days after illness onset varied from 8% to 13%, with no difference between children and adults (De Serres 2010). None of the subjects were shedding virus on day 11.
Another study conducted during an outbreak at the US Air Force Academy showed that 24% of 29 swabs collected on day 7 and 13% of the 16 swabs collected on day 8 of illness were culture positive, despite the large proportion of patients prescribed antiviral drugs (Witkop 2010).
A study in Singapore showed that the mean duration of viral shedding from illness onset was 6 days (plus or minus 2 days). Viral shedding persisted beyond 7 days, however, in 37% of patients. When prescribed during the first 3 days of illness, oseltamivir shortened the duration of viral shedding (Ling 2010).

Since viral shedding drops substantially after the first few days of influenza illness, the CDC recommends that persons with influenza-like illness should remain away from others until 24 hours after fever has resolved. In healthcare settings, the exclusion period should be continued for 7 days from symptom onset or until the resolution of symptoms, whichever is longer (CDC 2009: CDC recommendations for the amount of time persons with influenza-like illness should be away from others). More stringent guidelines and longer periods of exclusion may be necessary in some situations, if exposure to persons at high risk of complications is anticipated.

Groups at High Risk for Serious Illness and Complications

The groups at risk for complications related to seasonal influenza also have been shown to be at increased risk for complications following pH1N1 2009 influenza (Carcione 2010, Louie 2009, Writing Committee of the WHO Consultation on Clinical Aspects of Pandemic H1N1 2009 Influenza):

Pregnant women (especially those in the second or third trimester); pregnant women represent approximately 1% of the US population, but they accounted for 5% of deaths in the United States from pH1N1 2009 influenza reported to the CDC during the primary pandemic period (Siston 2010)

Persons aged 65 or older (the risk of infection in this group has been lower than for other age-groups, but if infection occurs, people of this age-group are at increased risk of complications)
Children <5 years old (Libster 2010)
Adults and children who have chronic pulmonary, cardiovascular, renal, hepatic, hematological, neurologic, or metabolic disorders
Adults and children who have immunosuppression (associated with HIV infection, organ transplantation, chemotherapy, corticosteroid use, or malnutrition)
Children and adolescents (age 6 months to 18 years) who are receiving long-term aspirin therapy and who might be at risk for experiencing Reye's syndrome after influenza virus infection

In addition, severe or morbid obesity (ie, a body mass index of 40 or more) has been found to be an important risk factor for complications or severe illness related to pH1N1 2009 infection (Kumar 2009, Louie 2009, Morgan 2010: Morbid obesity as a risk factor). This finding had not been noted previously with seasonal influenza.

Rates of severe disease also have been shown to be higher in indigenous populations in North America and the Pacific region (Kumar 2009, ANZIC Influenza Investigators 2009).


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