We conducted a longitudinal community cohort study of healthy adults in the UK. We found significantly higher incidence of influenza A(H1N1)pdm09 infection in 2010-11 than in 2009-10, a substantial proportion of subclinical infection, and higher risk for infection during 2010-11 among persons with lower preinfection antibody titers.
Keywords: England; United Kingdom; adult; cohort studies; epidemiology; human influenza; incidence; influenza A(H1N1)pdm09; natural infection; pandemic influenza; pandemics; seroepidemiology; seroprevalence; viruses.