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

Context: Data are limited and conflicting regarding the effectiveness of influenza 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 consecutive years, from 1992-1993 to 1994-1995. Setting: Two large teaching hospitals in Baltimore, Md. Participants: Two hundred sixty-four hospital-based health care professionals without chronic medical problems were recruited; 49 participated for 2 seasons; 24 participated for 3 seasons. The mean age was 28.4 years, 75% were resident physicians, and 57% were women. Intervention: Participants were randomly assigned to receive either an influenza vaccine or a control (meningococcal vaccine, pneumococcal vaccine, or placebo). Serum samples for antibody assays were collected at the time of vaccination, 1 month after vaccination, and at the end of the influenza season. Active weekly surveillance for illness was conducted during each influenza epidemic period. Main Outcome Measures: Serologically defined influenza infection (4- fold increase in hemagglutination-inhibiting antibodies), days of febrile respiratory illness, and days absent from work. Results: We conducted 359 person-winters of serologic surveillance (99.4% follow-up) and 4746 person- weeks of illness surveillance (100% follow-up). Twenty-four (13.4%) of 179 control subjects and 3 (1.7%) of 180 influenza vaccine recipients had serologic evidence of influenza type A or B infection during the study period. Vaccine efficacy against serologically defined infection was 88% for influenza A (95% confidence interval [CI], 47%-97%; P = .001) and 89% for influenza B (95% CI, 14%-99%; P = .03). Among influenza vaccinees, 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 controls (P = .41). Conclusions: Influenza vaccine is effective in preventing infection by influenza A and B in health care professionals and may reduce reported days of work absence and febrile respiratory illness. These data support a policy of annual influenza vaccination of health care professionals.