Influenza Vaccine Prevents Medically Attended Influenza-Associated Acute Respiratory Illness in Adults Aged ≥50 Years

Background. There are few estimates of effectiveness influenza vaccine in preventing serious outcomes due to influenza in older adults. Methods. Adults aged >= 50 years who sought medical care for acute respiratory illness were enrolled. A nose/throat swab was tested for influenza virus by reverse transcription-polymerase chain reaction. Clinical and demographic data were collected, including verification of receipt of trivalent inactivated influenza vaccination (IIV-3). Adjusted odds ratios were estimated by multivariable logistic regression models with an L1 penalty on all covariates except vaccination status. Results. A total of 1047 subjects were enrolled from November through April during 5 influenza seasons during 2006-2012, excluding the 2009-2010 season. Of those enrolled, 927 (88%) had complete influenza virus testing, vaccination status, and demographic data obtained. Of 86 (9.3%) influenza virus-positive patients, 47 (55%) were vaccinated. Of 841 influenza virus-negative patients, 646 (76.8%) were vaccinated. Over 5 influenza seasons, IIV-3 was 58.4% effective (95% confidence interval [CI], 37.0%-75.6%) for the prevention of medically attended laboratory-confirmed influenza illness in adults aged >= 50 years and 58.4% effective (95% CI, 7.9%-81.1%) in adults aged >= 65 years. Conclusions. Influenza vaccine was moderately effective in preventing influenza-associated medical care visits in older adults.