Pooled crude and adjusted PIVE.

aExcluding individuals with missing values.bStudy site included in the model as fixed effect.cExact logistic regression estimates with zero cases vaccinated.dModel adjusted for 2009–2010 seasonal influenza vaccination, any influenza vaccination in previous two seasons, presence of at least one chronic disease, sex, at least 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 symptom onset (note: in the 15–64 y stratum no adjustment for age group; in the “no chronic disease” stratum no adjustment for chronic disease or hospitalizations for chronic disease).eIf one of the cases would have been vaccinated, the estimated PIVE would be 85.2% (95% CI 30.0–98.3).fMissing data imputed using imputation using chained equations.