Influenza Update -- United States INFLUENZA VACCINE EFFICACY IN
NURSING HOME OUTBREAKS REPORTED DURING 1981-1982
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.
Barker WH, Mullooly JP. Influenza vaccination of elderly
persons. Reduction in pneumonia and influenza hospitalizations
and deaths. JAMA 1980;244:2547-9.
CDC. Influenza Surveillance Report No. 92. Issued January
1981,
p 35.
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.
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.
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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