Induction of Cross-Reactive Antibodies to 2009 Pandemic H1N1 Influenza Virus (pH1N1) After Seasonal Vaccination (Winters 2003/04 and 2007/08)

We measured haemagglutination inhibiting (HI) serum antibody titers to vaccine matched A/H1N1 influenza virus strain and to the new pandemic 2009 A/H1N1 (pH1N1) virus in two groups of volunteers prior and after 2003/2004 or 2007/2008 influenza seasonal vaccine administration. The responses were examined considering the overall volunteers studied in the two winters (144 and 79, respectively) and grouping those subjects in birth cohort classes (1903–1919; 1920–1957; 1958–1977). Before vaccination, HI antibody titers were found in all the groups examined and, on comparing the different age-groups, titers were higher in the younger groups as compared with the oldest against the A/H1N1 seasonal strains but titers were higher in the oldest as compared with the younger ones against the pH1N1 strain. Vaccination induced significant increases in HI titers against the matched A/H1N1 vaccine strains in all the groups examined. The responses satisfied the EMEA criteria and were higher in the youngest volunteers as compared with older groups. Increases were also found in the level of cross-reactive HI antibodies to the new pandemic 2009 A/H1N1 virus although in most instances the requirements of the EMEA were not met.