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Outbreaks of influenza A (H3N2, A/Shanghai/11/87-like) occurred in two partially (60% and 79%) vaccinated nursing home populations in January i988. A retrospective cohort study using chart review was designed to assess the effectiveness of influenza vaccination and amantadine prophylaxis (100 mg per day) in controlling the outbreaks and to determine the amantadine susceptibility of influenza viruses isolated from casepatients. The point estimate of vaccine efficacy in preventing influenza-like illness was −33% (95% confidence interval −115% to 18%). However, 9% of vaccinated case-patients died within 14 days after onset of influenza-like illness compared with 26% of unvaccinated case-patients (relative risk = 0.4, 95% confidence interval 0.1–1.0). There was no signfficant difference in illness severity among case-patients who became ill before amantadine prophylaxis was started (n = 84) compared …