Epidemiology of influenza in the elderly: evidence of virus recycling.

During the Hong Kong influenza epidemic of 1968-1969 the attack rate among persons born prior to 1890 was about one-third the rate among persons born after 1899. During an Asian influenza epidemic in the same community two years earlier there was no apparent age effect. Most persons born before 1890 had detectable pre-epidemic antibody against A/Hong Kong/68 whereas most persons born after 1899 had no detectable pre-epidemic antibody. The lower attack rate in the older age group is consistent with the hypothesis that influenza A viruses with similar or identical hemagglutinins can reemerge over time and that remote previous experience with the same or a similar strain can be protective. Also, no age group born after 1890 was found to have a high prevalence of antibody against A/England/72 or A/Dunedin/73, the successors to A/Hong Kong/68, indicating that recycling of hemagglutinin antigens is not strictly ordered.