In the US it was found that cross-reactive antibodies were more prevalent in those older than 60 years of age, while in Finland only those 80 years and older had high level of preexisting cross reactive antibodies. As only a few serum samples from individuals older than 80 years were included among the pre-pandemic sample in our study, we cannot exclude, that individuals who were born in the years after 1918 have higher pre-existing cross-reactive antibodies. The presence of cross-reactive antibodies in other age groups varies also between different studies. In Italy,Liarozole dihydrochloride and Australia there is some level of pre-existing cross-reactive antibodies found among all age groups with a common trend of higher proportions among older individuals, while in Finland, Norway, and US there was only little evidence of cross-reactive antibodies in other age groups than elderly. In Hong Kong only minor levels of pre-pandemic cross-reactive antibodies in the population with no age-specific trend is reported. These differences might be related to the methodological differences in the type and period of sample collection. We analysed samples collected over a 6 month-period directly prior to the start of the pandemic in Germany; in Italy samples from 2003– 2004 and in Finland samples from 2004–2005 were analysed. Our analysis stratified by three and six age groups, respectively,CID 1375606 suggest that also recently circulating H1N1 strains and vaccination history might have influenced the level of crossreactive antibodies in German adult population. The last season when seasonal H1N1 influenza viruses dominated in Germany and Europe was 2000/2001 and co-dominated in 2007/2008. Variation in the epidemiology of circulating subtypes between countries might also explain the different findings in the seroprevalence studies. This has to be taken into account when comparing serological results. The sources of sera also differ among the studies. In the UK, 1403 samples from the patients accessing health care were analysed and in Italy 587 samples were obtained from a seroepidemiological study.