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Four outbreaks of influenza-like illness in nursing homes, from which influenza type B virus was isolated and serologic evidence of influenza B infection was obtained, have been reported to CDC this winter (1-3). In all of these nursing homes the residents were elderly, with an average age in excess of 80 years. Three of the homes had resident populations of about 120 persons, but 1 institution had just over 40 residents.

Clinical attack rates for influenza averaged 27% (range 25% to 38%), and the duration of the outbreaks averaged 2 weeks (range 1-4 weeks). The percentage of recently vaccinated residents ranged from less than 10% to almost 90%. In 2 of the nursing homes, there were relatively equal numbers of vaccinated and unvaccinated residents. Laboratory diagnosis confirmed influenza B infections for 21 of 34 (62%) and 7 of 13 (54%) of ill residents from whom specimens were collected in these 2 homes, and the calculated rates of vaccine efficacy in preventing clinical influenza illness were 37% and 25%. Mortality associated with influenza-like illness among vaccinated and unvaccinated persons during the 4 outbreaks reported this winter led to an overall case-fatality ratio of 10%, but the total sample size was such that meaningful analysis of vaccine efficacy in reducing mortality was not feasible. Reported by Influenza Branch, Div of Viral Diseases, Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: The occurrence of several influenza B outbreaks involving nursing home residents, with some associated deaths, illustrates that even during a relatively mild influenza season, such as this year, when virus activity had been predominantly detected among school children, elderly persons are at risk of having moderate to severe influenza infections.

Previous studies have demonstrated that through vaccination it is possible to achieve an approximately 60% to 80% reduction of influenza morbidity among high-risk elderly persons, either living in open (4) or closed (5,6) populations. The reports from this year, however,as well as retrospective investigations of influenza A or B outbreaks during the past several years (7,8) suggest that attack rates can be high for vaccinated nursing home residents, and that the efficacy of current influenza A or B vaccines in reducing influenza morbidity among these persons may be lower than desired. The overall significance of such retrospective investigations is difficult to evaluate. Each of the study groups was selected through passive reporting, possibly introducing bias in selecting only outbreaks in which vaccine efficacy was poor. Accurate vaccine efficacy calculations also depend on several factors, including the validity of the case identification, and the medical basis for vaccine use. There may be many reasons, therefore, why retrospective studies in nursing homes suggesting that vaccine efficacy is low might not be representative of nursing homes in general. To determine whether artifacts of surveillance and investigation substantially affect the validity of conclusions about vaccine efficacy for nursing home residents will require further investigation.


  1. CDC. Influenza Update-United States, MMWR l982;31:131-2.

  2. CDC. Influenza Update-United States, MMWR l982;31:140-1.

  3. CDC. Influenza Update-United States, MMWR 1982;31:175-6.

  4. Barker WH, Mullooly JP. Influenza vaccination of elderly persons. Reduction in pneumonia and influenza hospitalizations and deaths. JAMA 1980;244:2547-9.

  5. CDC. Influenza Surveillance Report No. 92. Issued January 1981, p 35.

  6. Aymard M, Bentejac MC, Lambright G, Michaut D, Triau R. Efficacy of the antiinfluenza A vaccination during epidemics due to A/Victoria/3/75 and A/Texas/1/77 viruses. Devel. biol. Stand. 43:231-9.

  7. Hall WN, Goodman RA, Noble GR, Kendal AP, Steece RS. An outbreak of influenza B in an elderly population. J Inf Dis 1981;144:297-302.

  8. Goodman RA, Orenstein WA, Munro TF Smith SC, Sikes RK. Impact of influenza A in a nursing home. JAMA 1982;247:1451-3.

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