Characterization of the immune response to trivalent influenza vaccine in elderly men.

This study characterizes the time course of the immune response to influenza vaccine in elderly men. Sixty-two men aged 58 to 91 years (mean, 74.3 years) were vaccinated with trivalent inactivated influenza vaccine in the fall of 1983. Serum hemagglutinin-inhibiting (HAI) antibody titers were measured at various times up to 24 weeks postvaccination. Seroconversion frequencies determined at single times after vaccination were 28 to 46% of subjects, whereas cumulative seroconversion frequencies were greater than or equal to 70%. Eighteen to 28% of seroconversions occurred later than four weeks, and greater than or equal to 68% of those who seroconverted experienced greater than four-fold declines in peak HAI antibody titers by 24 weeks after vaccination. Consequently, 31 to 73% of subjects had HAI antibody titers greater than 40 throughout the study period. Measurement of HAI antibody titers at only one time after vaccination may not adequately reflect the immune response of this population or the degree of protection maintained through the influenza season.