Post-peak seroepidemiological studies of pandemic influenza (H1N1-2009) among a general population.

†Subjects, sample size and sampling period refer to those after observing the peak incidence of H1N1-2009. For several studies examining pre-existing immunity, the same or additional samples before the 2009 pandemic were investigated at different time periods, but are not included in this Table.‡Estimated proportions seropositive before and after observing an epidemic peak. When age-standardized estimate was given in the original study, we used it as the population mean.*Three studies did not estimate the proportion seropositive before the 2009 pandemic, and we assume that 7.5% of the population was initially immune based on a crude average among other studies.§After peak column represents if the sampling took place longer than 1 month after observing the highest incidence of cases.¥Vaccination column represents if a population-wide vaccination campaign of H1N1-2009 took place prior to the sampling.¶Laboratory methods to determine seropositivity; HI, hemagglutination inbibition assay and MN, microneutralization assay.