Evaluation of inter-pandemic influenza vaccine effectiveness during eight consecutive winter seasons in England and Wales in patients with cardiovascular risk factors

We evaluated the effectiveness of inactivated influenza vaccine in persons aged ≥45 years with co-morbidities (including essential hypertension) likely to have prompted consideration of prophylactic statin therapy. Using case-control analyses, we measured the impact of vaccination on General Practitioner (GP) reported new episodes of illness for clinically diagnosed influenza-like illness (ILI) and total acute respiratory infection (TARI) during eight consecutive winters, adjusting for potential confounders using multivariable techniques. Although the study failed to demonstrate the effectiveness of influenza vaccine, we identified important potential confounding related to a greater likelihood of vaccinees than non-vaccinees to consult with a GP for respiratory infections. Future researchers should consider and account for this phenomenon.