Effectiveness of influenza vaccine in health care professionals - A randomized trial

Context Data are limited and conflicting regarding the effectiveness of inf luenza vaccine in health care professionals. Objective To determine the effectiveness of trivalent influenza vaccine in reducing infection, illness, and absence from work in young, healthy health care professionals. Design Randomized, prospective, double-blind, controlled trial over 3 conse cutive years, from 1992-1993 to 1994-1995, Setting Two large teaching hospitals in Baltimore, Md, Participants Two hundred sixty-four hospital-based health care professional s without chronic medical problems were recruited; 49 participated for 2 se asons; 24 participated for 3 seasons. The mean age was 28.4 years, 75 % wer e resident physicians, and 57% were women. Intervention Participants were randomly assigned to receive either an influ enza vaccine or a control (meningococcal vaccine, pneumococcal vaccine, or placebo). Serum samples for antibody assays were collected at the time of v accination, 1 month after vaccination, and at the end of the influenza seas on. Active weekly surveillance for illness was conducted during each influe nza epidemic period. Main Outcome Measures Serologically defined influenza infection (4-fold inc rease in hemagglutination-inhibiting antibodies), days of febrile respirato ry illness, and days absent from work. Results We conducted 359 person-winters of serologic surveillance (99.4% fo llowup) and 4746 person-weeks of illness surveillance (100% follow-up). Twe nty-four (13.4%) of 179 control subjects and 3 (1.7%) of 180 influenza vacc ine recipients had serologic evidence of influenza type A or B infection du ring the study period. Vaccine efficacy against serologically defined infec tion was 88% for influenza A (95% confidence interval [CI], 47%-97%; P = .0 01) and 89% for influenza B (95% CI, 14% -99%; P = .03). Among influenza va ccinees, cumulative days of reported febrile respiratory illness were 28.7 per 100 subjects compared with 40.6 per 100 subjects in controls (P = .57) and days of absence were 9.9 per 100 subjects vs 21.1 per 100 subjects in c ontrols (P = .41). Conclusions Influenza vaccine is effective in preventing infection by influ enza A and B in health care professionals and may reduce reported days of w ork absence and febrile respiratory illness. These data support a policy of annual influenza vaccination of health care professionals.