Many serologic studies were done during and after the 2009 influenza pandemic, to estimate the cumulative incidence of influenza A(H1N1)pdm09 virus infections, but there are few comparative estimates of the incidence of influenza A(H3N2) virus infections during epidemics.

We conducted a longitudinal serologic study in Hong Kong. We collected sera annually and tested samples from 2009-13 by HAI against the A/Perth/16/2009(H3N2) virus, and samples from 2013-15 against the A/Victoria/361/2011(H3N2) virus using the hemagglutination inhibition (HAI) assay. We estimated the cumulative incidence of infections based on 4-fold or greater rises in HAI titers in consecutive sera.

We found that re-infections in each of the four H3N2 epidemics that occurred from 2010 through 2014 were rare. The largest H3N2 epidemic occurred with the lowest level of pre-epidemic immunity.

There were four major H3N2 epidemics: (1) Aug-Oct 2010; (2) Mar-Jun 2012; (3) Jul-Oct 2013; and (4) Jun-Jul 2014. Between 516 and 619 relevant pairs of sera were available for each epidemic. We estimated that 9%, 19%, 7% and 7% of the population were infected in each epidemic, respectively, with higher incidence in children in epidemics 1 and 4.