The decline of the peak in children might partly be explained by immunity in the youngest age groups, possibly related to high attack rates of influenza A(H1N1) 2009 virus in children during the pandemic season or to persisting vaccine-induced immunity [14] [15] [16] . Although the infection attacks rates during the pandemic season were very low in the older adults (≥ 40 years), the shift of the peak in mortality rates towards older age groups observed in our study might indicate increased circulation of the virus in the 2010-2011 influenza season in these age groups [14] . A shift of the age-specific mortality pattern similar to that observed in our study is also described for the post-pandemic seasons following the three pandemics in the 20 th century. During each of these earlier pandemics, persons younger then 65 years of age initially accounted for a high proportion of influenza-related deaths, followed by a declining proportion of deaths in the postpandemic seasons [17] . Simonsen et al. [17] hypothesised that younger persons may retain long-lasting immunity better than older persons after exposure to a new influenza virus subtype.