A meta-analysis of effectiveness of influenza vaccine in persons aged 65 years and over living in the community
Introduction
Although influenza affects all age groups, persons with underlying medical conditions and persons aged 65 years and over are at greatest risk of severe illness and possibly death [1], [2]. As recommended by the World Health Organization, the major strategy for controlling influenza is annual immunization of those at greatest risk with an inactivated influenza vaccine [3].
However, published studies on influenza vaccine effectiveness (VE) have produced inconsistent results. The only randomized double-blind placebo-controlled trial, conducted in The Netherlands in the 1991–1992 influenza season, found that the vaccine reduced serologically-confirmed influenza by 50% (95% CI 39–65%) [4]. However, the reduction was only 23% (95% CI 51–61%) in persons 70 years of age and over. One cohort and two case-control studies have reported low estimates of VE with wide confidence intervals, including the possibility that the vaccine had no effect [5], [6], [7].
A previously published meta-analysis of influenza VE in the elderly concentrated largely on residents of nursing homes [8]. The only meta-analysis of influenza VE in persons aged 65 years and over living in the community is a Spanish language publication [9] limited to one influenza-related outcome. With the aim of estimating influenza VE for all major influenza-related outcomes for persons aged 65 years and over living in the community, we performed a comprehensive and systematic review of the available literature and a meta-analysis on studies that satisfied specified inclusion and exclusion criteria.
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Methods
We followed guidelines for preparing systematic reviews and meta-analyses, using multiple sources of information without language restriction [10], [11], [12]. The cut-off date for the literature search was the 31 December 2000. Differences in the effectiveness of whole virion, subunit virion and split virion vaccines were outside the scope of this study. Additionally, we excluded studies examining differences between single and multiple vaccinations, influenza vaccine immunogenicity or
Studies included in the meta-analysis
Database and website searches identified 49 relevant publications, including 12 publications in languages other than English. Of the latter group, six satisfied the inclusion/exclusion criteria and were translated by qualified translators [9], [17], [18], [19], [20], [21]. No additional publications or unpublished studies were identified by manual searches or by the two prominent researchers in the field.
We excluded 34 of the 49 publications. Table 3 summarizes nine studies excluded for reasons
Discussion
Results of this meta-analysis confirm that the influenza vaccine is effective in reducing influenza-related illness and death among persons 65 years and over living in the community. When there is a good match between influenza strains in the vaccine and those in circulation, vaccination would prevent approximately one in five cases of influenza-like illness, one in four hospitalizations for pneumonia and influenza and one in four deaths following hospitalization for these conditions. These
Acknowledgements
We would like to thank Dr. J.S. Nguyen-Van-Tam from the University of Nottingham and Dr. Kristin L. Nichol from the Veterans Affairs Medical Center (Minneapolis) for their comments on the bibliography. We are indebted also to Mr. Shigeru Takahashi and Ms. Qui Nguyen for their translation of a study written in Japanese, and to Dr. David Fairservice for his translation of Spanish and Italian articles. We thank Julianne Lynch for her helpful suggestions during the preparation of the manuscript.
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