Age distribution of patients with medically-attended illnesses caused by sequential variants of influenza A/H1N1: comparison to age-specific infection rates, 1978-1989.

Since influenza A/H1N1 viruses reappeared during the 1977-1978 season, this subtype has contributed 27% of 6,609 documented influenza infections of persons with acute respiratory disease presenting to clinics serving as surveillance sites of the Influenza Research Center in Houston for the 12-year period ending June 1989. Wide differences in the distribution of H1N1 viruses occurred by age group: more than 50% of H1N1 infections were detected among persons aged 10-34 years, compared with 28% for influenza A/H3N2 and 35% for influenza B. Over age 35 years, the contribution of H1N1 viruses dropped to only 4%, compared with 20% and 16% for influenza A/H3N2 and influenza B, respectively. When birth dates of persons with positive cultures were examined, it was found that most of the H1N1-positive persons were born after 1950. Concurrently, longitudinal studies of families and other adults under intensive surveillance for infection, including cultures of all respiratory illnesses and tests for serum antibody rise over the respiratory disease season, revealed appreciable infection rates for adults born before 1950. Furthermore, the annual peak of hospitalization of older persons with pneumonia and other acute respiratory illnesses was significantly correlated with the peak of H1N1 virus activity in 1978-1979, a year when H1N1 viruses were the only influenza viruses prevalent. These observations indicate that many persons infected with influenza A/H1N1 viruses that circulated from 1946 through 1953 have immunity which has persisted for more than 25 years but this immunity is not complete. Reinfection that may result in serious illness in older vulnerable adults does occur but with lower frequency than with influenza A/H3N2 infection. Currently prevalent H1N1 variants are antigenically different from those that circulated in the 1950s; however, older adults readily acquire immunity to these new variants--perhaps as a result of immunologic priming that occurred in childhood.