Fig. 1.

Study outline. Four groups of healthcare workers (Group 1; G1, Group 2; G2, Group 3; G3 and Group 4; G4) were retrospectively selected according to their year of birth (G1; 1947–1952, G2; 1958–1960, G3; 1969–1972 and G4; 1978–1980), in relation to circulating influenza subtypes. The dominating influenza A subtypes circulating from 1918 and prior to 2009 are shown. All healthcare workers (n = 20 in each group) received the AS03 adjuvanted monovalent pandemic influenza vaccine in 2009. Serum samples were collected before vaccination (D0) and 21 days (D21) after vaccination.