The study, will use four primary data sources: the General Practice Research Database (GPRD, for mild outcomes), the Hospital Episode Statistics database (HES, for hospitalizations), the Office of National Statistics mortality database (ONS, for mortality), and weekly virology data from the Health Protection Agency (HPA).

Weekly time series of the rates of various influenza-related health outcomes in the various databases, such as office visits for acute otitis media and hospitalizations for pneumonia will be constructed. Statistical models, guided by weekly numbers of cases of laboratory-confirmed influenza and respiratory syncytial virus (RSV) contained in the HPA virology data, to estimate the portions of the various outcomes that can be attributed to influenza will be constructed. Finally, the seasonal impact of influenza by risk status, vaccination status, and seasons in which a well-matched versus mismatched influenza vaccine was used, will be assessed.