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.