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Flu vaccines prevent infection in younger adults. But do they prevent illness and death in older people—the largest target group? A large observational study of data from three health maintenance organisations (HMOs) in the US shows that during 10 flu seasons in the 1990s, vaccination of older people was associated with a 27% reduction in hospital admissions for pneumonia or flu (adjusted odds ratio 0.73, 95% CI 0.68 to 0.77) and a 48% reduction in deaths (0.52, 0.50 to 0.55)0.55).
The authors considered the possibility that vaccinated people were fitter or looked after themselves better than unvaccinated people, but they found no evidence of a “healthy vaccinee effect.” Benefits were smaller in seasons when the available vaccine didn't match the circulating strain of virus particularly well. This suggests that the observed effects are real and not simply the result of confounding, says a linked editorial (p 1439).
So flu vaccines work for older people living at home in the US. But they could work better. The immune system weakens with age, and more immunogenic vaccines are needed to compensate, says the editorial. Better coverage would also help. In this study, only 58% of HMO members over 65 were vaccinated.