Figure 1. Schematic representation of timing of epidemics, study follow-up and vaccinations.

Time course of influenza epidemics in Finland according to the National Infectious Disease Registry (NIDR) and the follow-up of study participants during the epidemic seasons 2009–10 (panel A) and 2010–11 (panel B). Note differences in scales. The main circulating influenza A virus during both epidemic seasons was A(H1N1)pdm09; in 2009–10 only sporadic H3N2 and few influenza B virus findings were observed and during 2010–11>95% of the influenza A strains were A(H1N1)pdm09. The figure shows the vaccination times of the study participants with Pandemrix and seasonal trivalent influenza vaccine (TIV) in 2010–11. The national prioritization order for pandemic vaccination (here shown for adults) in 2009–10 was determined on medical grounds.