Pooled crude and adjusted 2009–2010 seasonal VE, multicentre case-control study, influenza season 2009–2010, seven European Union study sites.

aExcluding individuals with missing values.bStudy site included in the model as fixed effect.cModel adjusted for 2009–2010 pandemic influenza vaccination, any influenza vaccination in previous two seasons, presence of at least one chronic disease, sex, at list one hospitalization for chronic disease in the previous 12 mo, current smoker, age group, practitioner visits in previous 12 mo (0, 1–4, and 5+ visits), month of symptoms onset.dMissing data imputed using imputation using chained equations.