Naturally acquired immunity to influenza type A: a further prospective study.

During the 1976 influenza epidemic, the incidence and severity of attack by A/Victoria/3/75 strain were studied in 312 participants who were divided into two groups: in Group 1 were 216 participants who had a history of laboratory-proven infection by one of the earlier strains of Hong Kong subtype; in Group 2 were 96 participants who had no known history of such prior infection. No participant in either group received influenza vaccine. The efficacy of clinical immunity, acquired as a result of prior infection by an earlier Hong Kong subtype strain, was judged by comparing the findings in Group 1 and Group 2. Incidence of laboratory-proven A/Victoria/3/75 infection was found to be 27% in Group 2 (26 cases confirmed) and 10.6% in Group 1 (23 cases confirmed). Among Group 1 members, the incidence was highest in those with the longest interval since prior Hong Kong subtype infection. Severe influenzal illness was seen in 14 of the 26 proven cases in Group 2, and in only three of the 23 proven cases in Group 1. The prior infection in these three had been by A/Hong Kong/1/68 strain in 1969-1970. Thus in Group 1, severe influenzal illness was not observed until six years or more had passed since prior infection (and three episodes of antigenic drift had intervened). Results suggested that low serum antibody levels may be unreliable as indicators of clinical immunity. Preepidemic sera of 99 members of Group 1 and 82 members of Group 2 showed no HAI antibody to A/Victoria/3/75 (titre less than 8), but the incidence of laboratory-proven A/Victoria/3/75 infection among these individuals was lower in Group 1 (19%) than in Group 2 (30.5%).