An outbreak of influenza B at an Indiana boarding school: estimate of vaccine efficacy.

During the influenza season of 1979-80, influenza B accounted for most cases of influenza in the United States. The midwestern States experienced widespread influenza B activity beginning in mid-December. From late January to mid-February an outbreak of influenza-like illness occurred at a private boarding school for boys in northeastern Indiana. Most of these boys had been vaccinated against influenza. An influenza B virus was isolated in 1 instance, and 5 of the 24 boys from whom acute- and convalescent-phase serum specimens were obtained showed evidence of influenza B infection.Through a questionnaire survey, it was determined that 18 percent of the boys had had an influenza-like illness; the attack rate was 39 percent for the unvaccinated and 14 percent for the fully vaccinated. The vaccine efficacy was thus estimated to be 63 percent. As was true of many of the influenza B viruses isolated in the 1979-80 influenza season, the virus isolated at the school demonstrated some variation from the B/Hong Kong/5/72 virus used in the vaccine.