Repeated seasonal influenza vaccination among elderly in Europe: Effects on laboratory confirmed hospitalised influenza

Our results suggest that, regardless of patients' recent vaccination history, current seasonal vaccine conferred some protection to vaccinated patients against hospitalisation with influenza A(H3N2) and B. Vaccination of patients already vaccinated in both the past two seasons did not seem to be effective against A(H1N1)pdm09. To better understand the effect of repeated vaccination, engaging in large cohort studies documenting exposures to vaccine and natural infection is needed.