However, influenza vaccine failure is common even during seasons with optimal antigenic match between circulating and vaccine viruses. Among adults, vaccine efficacy in preventing laboratory confirmed LEE011 datasheet influenza illness is estimated to be approximately 60% [3]. Similar efficacy has been reported for preventing hospital admission with laboratory confirmed pandemic or seasonal influenza [4], [5], [6], [7], [8], [9] and [10].

It is not clear if influenza vaccination prevents serious outcomes by primary prevention of influenza infection, by reducing severity of influenza illness, or both. We conducted a population based study of laboratory confirmed influenza among adults aged ≥20 years over multiple seasons to determine if receipt of same-season influenza vaccine was associated with reduced risk of hospital admission within 14 days after onset of influenza illness. This was a secondary analysis of data from ABT-888 solubility dmso population-based studies of influenza vaccine effectiveness during eight influenza seasons, 2004–05 through 2012–13, in Marshfield, Wisconsin [11], [12], [13] and [14]. In this community, residents receive nearly all outpatient and inpatient care from the Marshfield Clinic. A single acute care hospital (St. Joseph’s) serves the study population, and both inpatient and outpatient diagnoses are accessible through a combined electronic medical record. The electronic

medical record captures 90% of outpatient visits, 95% of hospital discharges, and 99% of deaths for the residents in the area [15], [16], [17] and [18]. During each influenza season, eligible community dwelling residents were recruited by trained research coordinators during or after an inpatient or outpatient medical encounter for acute respiratory illness. Research coordinators used an electronic appointment system to identify and recruit eligible persons

in all primary care clinics and in urgent care on weekdays, evenings, and weekends. Eligible persons were also recruited at the hospital that is contiguous with Marshfield Clinic. Most ill persons who were not approached during a clinical encounter were identified on the following day by use of electronic diagnosis codes entered by attending physicians (ICD-9-CM codes 382.0, 382.4, 382.9, 460–466, 480, 483–486, 487, 490, 780.6, and 786.2). These individuals were contacted by telephone, to and a swab sample was obtained at home from those who were eligible and consented. Participants completed a short interview to assess illness symptoms and onset date; nasopharyngeal swabs were obtained for influenza testing. Real-time reverse transcription polymerase chain reaction (RT-PCR) and viral cultures were performed at the Marshfield Clinic Research Foundation as previously described [11]. Culture alone was performed on samples collected in 2004–05 and RT-PCR was performed in subsequent years. Subtype results based on RT-PCR were not available for 11% of influenza A positive samples.