Estimating the influenza vaccine effectiveness in elderly on a yearly basis using the Spanish influenza surveillance network—Pilot case–control studies using different control groups, 2008–2009 season, Spain

We conducted a case–control and screening method studies to estimate influenza vaccine effectiveness (IVE) in the age group ≥65 years, based on the Spanish Influenza Sentinel Surveillance System (SISSS). Cases (influenza laboratory-confirmed) were compared to influenza-negative ILI patients (test-negative) and patients without ILI since the beginning of the season (non-ILI). For the screening method, cases’ vaccination coverage was compared to the vaccination coverage of the GPs’ catchment population. The results suggested a protective effect of the vaccine against laboratory-confirmed influenza in elderly in 2008–2009. The screening method and the test-negative control designs enable estimating IVE using exclusively SISSS data.