Pre- and Postpandemic Estimates of 2009 Pandemic Influenza A(H1N1) Seroprotection to Inform Surveillance-Based Incidence, by Age, During the 2013–2014 Epidemic in Canada

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To understand the epidemic resurgence of influenza due to the 2009 pandemic influenza A(H1N1) strain (A[H1N1]pdm09) during the 2013–2014 influenza season, we compared age-related cross-sectional estimates of seroprotection before the pandemic (during 2009) and after the pandemic (during 2010 and 2013) to subsequent surveillance-based, laboratory-confirmed incidence of influenza due to A(H1N1)pdm09 in British Columbia, Canada. Prepandemic seroprotection was negligible except for very old adults (defined as adults aged ≥80 years), among whom 80% had seroprotection. Conversely, postpandemic seroprotection followed a U-shaped distribution, with detection in approximately 35%–45% of working-aged adults but in ≥70% of very old adults and young children, excluding children aged <5 years in 2013, among whom seroprotection again decreased to <20%. The incidence was 5-fold higher during 2013–2014, compared with 2010–2011, and was highest among children aged <5 years and working-aged adults, reflecting a mirror image of the age-based seroprotection data.

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