Objective: To study the incidence of H1N1 influenza from 1977 to 1988 in unvaccinated volunteers and the effects of continuing minor antigenic change (antigenic drift) in the virus. Design: Prospective study by a group of general practitioners, backed up by virological findings. Participants: Mainly patients of the general practitioner group, also some doctors and members of staff. There were 287 participants during 1977-1981, and 207 at the end of 1988. Intervention: Any participant deemed to be "at risk" was encouraged to be vaccinated and to withdraw from the study. Background: In 1957, H1N1 subtype influenza had been displaced by H2N2 (Asian) subtype. In 1968, H2N2 was displaced by H3N2 (Hong Kong) subtype. During 1977, H1N1 influenza unexpectedly reappeared in Asia, and spread widely. The resurgent strain, designated A/USSR/90/77(H1N1), caused world pandemics, attacking (almost exclusively) persons who had been born since the 1950-1951 northern winter and causing negligible mortality. It did not displace the current H3N2 strain, and strains of both subtypes have continued to emerge independently. Hypothesis: Antigens of A/USSR resembled closely those of the 1950-1951 H1N1 strain, which apparently was rendered antigenically inert between 1951 and 1976 (possibly frozen) and was reactivated during 1977. Antigenic drift was then resumed. Main outcome measure: The A/USSR/90/77 strain and its close successor, A/Brazil/11/78, attacked mainly the young, whose previous exposure to H1N1 antigens had been minimal or zero. Mortality during the A/USSR pandemics was negligible because death from influenza in people aged less than 30 years is rare. Would continuing antigenic drift ultimately widen the H1N1 spectrum of attack? Result: During the epidemic of 1988, A/Taiwan/1/86(H1N1) attacked a wider range of age groups than had A/USSR or A/Brazil. Conclusion: Assuming that H1N1 viruses continue to undergo further antigenic drift, an ever widening age spectrum of H1N1 attack may be expected.
