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—To assess the clinical effectiveness of influenza vaccination in preventing influenza-associated hospitalization and death.

—Case-control study.

—Noninstitutionalized persons aged 45 years or older living in Manitoba, on December 1, 1982, and December 1, 1985.

—Linked records of the Manitoba population registry, hospital-discharge abstracts, physician claims for ambulatory-patient visits and influenza vaccination, and vital statistics were used. A matched-set analysis estimated the clinical effectiveness of influenza vaccination in preventing hospital admissions and deaths from influenza-associated conditions during influenza A (H3N2) outbreak periods in 1982 to 1983 (12 weeks) and 1985 to 1986 (10 weeks). The analysis adjusted for hospital discharge and ambulatory care for high-risk conditions within the previous 15 months and 3 months, respectively.