In Finland, 1031 stored samples from hospital virology laboratory, in Hong Kong sera blood donors, hospital outpatients and community study and in Norway age- and geographically representative residual sera from hospital laboratories were analysed. Using samples collected for other purposes might lead to selection bias with overrepresentation of healthy young adults or persons with particular health problems. These groups might have different exposure to pH1N1 than general population as well as SR2595 SAK3 differences in immunological response. In our study we used a subsample from a population based representative nation wide survey. Another explanation for the variable results might be related to the differences in laboratory procedures; in our study – as well as the study in Finland, Norway and Australia – HI titres were determined, while some of the other studies used microneutralisation assays or both. Due to the problems of reproducibility of the HI as well as MN methods between laboratories the levels of detected antibody titres may differ among studies if methods are not standardized. Our comparison of reactive antibody prevalence against pH1N1 in pre and post-pandemic sera indicates that in Germany around one third of those aged 18–29 years and around one fifth of those 30–49 years of age were infected with 2009 pandemic influenza A. While those 50 years of age and older had no detectable increase in the proportions of reactive antibodies at titre. However, analysing individual titres using Tobit modelling with three birth cohorts adjusting for vaccination, we showed that those born before 1957 had a significant increase in the GMT, but that the increase was the smallest in the three birth cohorts. A similar study from Canada observed the lowest rate of titre $40 in those 50–79 years old after the pandemic. A study by Miller et al. found no measurable difference between pre- and postpandemic period in England among those 45 years and older. A study by Bandaranayake et al. describes higher infection rates in younger individuals and almost no measurable infection rates among elderly.