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

In Europe, annual influenza vaccination is recommended to elderly. From 2011 to 2014 and in 2015\u201316, we conducted a multicentre test negative case control study in hospitals of 11 European countries to measure influenza vaccine effectiveness (IVE) against laboratory confirmed hospitalised influenza among people aged 6565 years. We pooled four seasons data to measure IVE by past exposures to influenza vaccination. We swabbed patients admitted for clinical conditions related to influenza with onset of severe acute respiratory infection 647 days before admission. Cases were patients RT-PCR positive for influenza virus and controls those negative for any influenza virus. We documented seasonal vaccination status for the current season and the two previous seasons. We recruited 5295 patients over the four seasons, including 465A(H1N1)pdm09, 642A(H3N2), 278 B case-patients and 3910 controls. Among patients unvaccinated in both previous two seasons, current seasonal IVE (pooled across seasons) was 30% (95%CI: 1235 to 64), 8% (95%CI: 1294 to 56) and 33% (95%CI: 1243 to 68) against influenza A(H1N1)pdm09, A(H3N2) and B respectively. Among patients vaccinated in both previous seasons, current seasonal IVE (pooled across seasons) was 121% (95%CI: 1280 to 43), 37% (95%CI: 7\u201357) and 43% (95%CI: 1\u201368) against influenza A(H1N1)pdm09, A(H3N2) and B respectively. Our results suggest that, regardless of patients\u2019 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