Study designs for timely estimation of influenza vaccine effectiveness using European sentinel practitioner networks

European sentinel practitioner influenza surveillance networks represent a simple and feasible framework to conduct observational studies providing rapid and repeated influenza vaccine effectiveness estimates. The minimum requirements for those studies should be to correctly ascertain vaccination status, to include laboratory-confirmed influenza as outcome and to collect variables to control for confounding. Various study designs are possible including the screening method, computerised cohort and case control studies using various control groups. Selecting the study design is a compromise between methodological constraints, availability and ease of access to data sources and resources available. Results from practitioner-based studies complement other pieces of evidence (e.g. immunogenicity studies, vaccine efficacy, surveillance data) to assess the effect of influenza vaccination in the population.