Abstract
Understanding household transmission of the pandemic influenza A(H1N1)2009 virus, including risk factors for transmission, is important for refining public health strategies to reduce the burden of the disease. During the influenza season of 2009 we investigated transmission of the emerging virus in 595 households in which the index case was the first symptomatic case of influenza A(H1N1)2009. Secondary cases were defined as household contacts with influenza-like illness (ILI) or laboratory-confirmed influenza A(H1N1)2009, occurring at least one day after but within seven days following symptom onset in the index case. ILI developed in 231 of the 1,589 household contacts, a secondary attack rate of 14.5% (95% confidence interval (CI): 12.9–16.4). At least one secondary case occurred in 166 of the 595 households (a household transmission rate of 27.9%; 95% CI: 24.5–31.6).Of these, 127 (76.5%) households reported one secondary case and 39 (23.5%) households reported two or more secondary cases. Secondary attack rates were highest in children younger than five years (p=0.001), and young children were also more efficient transmitters (p=0.01). Individual risk was not associated with household size. Prophylactic antiviral therapy was associated with reduced transmission (p=0.03). The secondary attack rate of ILI in households with a confirmed pandemic influenza A(H1N1)2009 index case was comparable to that described previously for seasonal influenza.
Highlights
The world experienced the first influenza pandemic of the 21st century in 2009
Influenza is a notifiable disease in Australia, and cases were identified from the Western Australia (WA) Notifiable Infectious Diseases Database, which is maintained by the Communicable Disease Control Directorate (CDCD)
In order to estimate the proportion of influenzalike illness (ILI) cases due to pandemic influenza, we identified all secondary cases who had swabs collected within 48 hours of onset of ILI symptoms, at which time the yield should be optimal [17]
Summary
The world experienced the first influenza pandemic of the 21st century in 2009. Pandemic influenza has since dominated the 2009/10 northern hemisphere winter and the 2010 southern hemisphere winter. Understanding the transmission dynamics of pandemic influenza, including risk factors for transmission, is important in informing public health strategies to reduce the impact of the virus. Household transmission studies of the current [4,5,6], and previous influenza pandemics are scarce [7], and rely on studies of seasonal influenza [8,9,10,11,12]. Secondary attack rates reported for seasonal influenza range from 10% to nearly 40% and vary with age, circulating strain, family composition, and levels of community exposure [8,9,10,11,12]
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