Pooled crude and adjusted PIVE, according to categories based on delay between date of vaccination and date of onset of symptoms.

aStudy site included in the model as fixed effect.bModel adjusted for 2009–2010 seasonal influenza vaccine, 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.