This analysis excluded the 2009–10 season because monovalent vaccine was not available to the local population when the pandemic wave arrived in October–November

2009, and influenza was absent from the study population in the subsequent winter months. Influenza vaccination status was determined by a real-time, internet-based vaccination registry used by all public and private vaccination providers serving the population (http://www.recin.org). A validation study of PD0325901 the registry during the 2006–07 and 2007–08 influenza seasons demonstrated that the registry captured 95% of all influenza vaccinations that were received by study participants [19]. A similar high level of capture was demonstrated in a validation study during the 2011–12 season (unpublished data). Adults were classified as vaccinated if they had received influenza vaccine ≥14 days before the onset of illness. Dates of hospital admission and discharge diagnoses were identified from the electronic medical record for a 14 day period after onset of influenza illness. To adjust for use of antiviral drugs, we extracted dates of antiviral prescriptions for all participants. The main outcome was an acute care hospital admission occurring within 14 days of

influenza symptom onset. Although most hospital admissions occurred after an outpatient enrollment, some participants were initially enrolled and swabbed after admission to the hospital. Covariates included age, MK 2206 Rutecarpine gender, antiviral prescription, specific high risk

medical conditions, year, and influenza type/subtype [A/H3N2, A/H1N1, pandemic H1N1 (A/H1N1pdm09), B]. Study participants were classified as having a high risk medical condition if they had at least one visit during a recent 12 month period with an ICD-9 CM diagnosis code of interest. High risk conditions were classified into the following groups: cancer, cardiovascular disease, diabetes, pulmonary, and other. Antiviral prescription was defined as a prescription of oseltamivir, zanamivir, amantadine, or rimantadine within 14 days of symptom onset for persons not hospitalized and between symptom onset and hospital admission for persons who were hospitalized. We restricted the analysis of hospital admissions to enrolled adults aged ≥20 years because influenza-related hospitalization was less common in children, and potential confounding factors are likely to be different for adults and children. Studies of influenza vaccination and hospital admission are particularly susceptible to confounding, since persons who are vaccinated may be more likely to have pre-existing chronic medical conditions or other risk factors for hospital admission. To minimize confounding by indication for vaccination, we used a propensity score regression adjustment [20] and [21].