A recent study [12] has shown that immune protection naturally acquired during the first wave of the 1918 influenza pandemic provided mortality and morbidity protection during the successive pandemic wave. Several investigations have evaluated the frequency of the pre-existing cross-reactive antibodies and the acquisition of immunity in the pre-pandemic and post-pandemic (H1N1) 2009 periods [26–29]. These studies have found higher seropositivity in older patients (those born before 1957) and negligible or minimal seropositivity in younger patients in the pre-pandemic per
iod. By contrast, after the pandemic period, higher seropositivity against pandemic (H1N1) virus was found among the school-aged population and young adult patients (30 years developed lower immune protection capable of neutralizing pandemic influenza A (H1N1) 2009 virus. Our findings of an older age in hospitalized patients during the post-pandemic influenza season are therefore consistent with the seroprevalence studies mentioned above.