technical challenges with the assays, interpretation, and validation of results. Available serological data on prevalence or seroincidence of humoral immunity yielded age-specific attack rates that indicated a substantial proportion of asymptomatic infections and mild illnesses, similar to or greater than past pandemics and seasonal outbreaks. This was confirmed by a recent Hong Kong study showing the proportion of asymptomatic infection, secondary attack rates, viral shedding, and course of illness among household members were largely similar between infections with seasonal and pandemic influenza virus strains circulating during 2009 [11] . The few published serosurveys revealed heterogeneities in infection rates among different groups and between different places [12] [13] [14] . In particular there appears to be serological evidence of substantial preexisting humoral immunity among older adults, ranging from 23% (1:32 titre by haemagglutination inhibition in those 65 years or over) [14] to 34% (1:80 titre by microneutralisation assay in those 60 years or over) [15] in different studies. Further data on population susceptibility by age or the availability of a rapid and accurate serological test could allow health services to further target vaccine efforts for subsequent waves, as has been done in a few countries [14] .