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A/H3N2 HI antibody titer and estimated risk of A/H3N2 influenza disease in subjects aged ≥65 years in an epidemic of low to moderate intensity (A) or high intensity (B), and in subjects aged <65 years in an epidemic of low to moderate intensity (C) or high intensity (D). Dots represent the observed proportions of cases and shading shows 95% confidence interval (logistic regression). Abbreviation: HI, hemagglutination inhibition
