In the literature the presence of cross-reactive antibodies among the elderly as well as lower infection rates during the pandemic are explained by cross-reactive immunity due to previously circulating influenza A strains. The correlation between the HI and clinical protection has been documented for seasonal influenza viruses and HI titre in the range of 30–40 is generally accepted to be associated with a 50% reduction in the risk of influenza infection or disease in a population. In our study,Tasimelteon those over 50 years of age had lower proportions of pre-existing cross-reactive antibodies and at the same time lower infection rates. One of the possible reasons for lower risk of infection among older individuals could be pre-existing immunity not detectable by cross-reactive antibodies. This is supported by our results showing that those younger than 50 years of age had highest levels of cross-reactive antibodies prior pandemic as well as highest infection rates. This is in concordance with higher notification rates in those adults younger than 50 in comparison to those over 50 years of age. Other possible explanations are that the older age groups were possibly less affected by pH1N1 infection as they had less contact with younger age groups,Didox or that due to weaker immune response we observe lower reactive antibody levels among elderly. Moreover, infection and vaccination can induce T-cell mediated immune response in humans and it has been shown that some memory T-cell immunity against 2009 is present in the adult population. Our study has some key characteristics that the aforementioned studies lack. We analysed a representative sample set that was collected 6 months before the pandemic for the pre-pandemic analysis and right after the pandemic for the post-pandemic analysis. Due to the availability of vaccination cards, we were able to control for the effect of pandemic vaccination on measured antibody titres in the post-pandemic period. Moreover, our study is population-based, while other studies used samples from specific groups, e.g. blood donors or hospitalised persons. We believe that these characteristics are the major strengths of our investigation.