title: Naturally acquired immunity to influenza type A. Lessons from two coexisting subtypes.
abstract: When the H1N1 subtype of influenza reappeared in the Northern Hemisphere during 1977, after a 20-year absence, it behaved very differently from the H3N2 subtype still in circulation. In Sydney, we studied the incidence of both subtypes of laboratory-proven influenza type A in 287 unvaccinated volunteers whose serum antibody titres were measured before and after each winter, to facilitate the detection of subclinical as well as clinical infection. During a 1977 epidemic, the A/Victoria/3/75 strain of the H3N2 subtype attacked participants of all age groups, whereas during epidemics of 1979 and 1981, the A/USSR/90/77 and A/Brazil/11/78 strains of the H1N1 subtype attacked only subjects born after 1950. The older participants apparently possessed homologous protection, acquired as a result of exposure to H1N1 more than 20 years earlier and not dependent upon strain-specific haemagglutination inhibition antibody.
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D001938:Brazil; D002648:Child; D005260:Female; D006801:Humans; D007113:Immunity, Innate; D009980:Influenza A virus / Q000276:immunology; D007251:Influenza, Human / Q000453:epidemiology / Q000276:immunology* / Q000401:mortality; D008297:Male; D008875:Middle Aged; D014586:USSR; D006113:United Kingdom