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Although yearly influenza vaccination is recommended in the U.S. for elderly persons, only about 60% of this population is vaccinated — well short of the 90% goal. Estimates of influenza-vaccine effectiveness in the elderly are based mostly on short-term studies covering one or two influenza seasons at a single site.
To address issues of seasonal and geographic variability, as well as potential bias and residual confounding, investigators conducted a large-scale, multicenter, long-term study of vaccine effectiveness in community-dwelling individuals aged ≥65. Data concerning influenza vaccination status were pooled from three HMOs (one in Minnesota/Wisconsin for 1990–1991 through 1999–2000, and one each in Oregon/Washington and New York for …