Re-emergence of H1N1 in 1977
Support for theory of immunity was found in the1977 re-emergence of H1N1.
H1N1 was absent in human populations from 1957 to 1977. H1N1 strains collected in 1957 and 1977 were nearly identical, both antigenically and genetically, and the majority of illness occurred in individuals younger than 20 years, presumably because most older individuals retained H1N1 immunity from the 1950s.