A total of 663 pupils at four schools were studied serologically and clinically during a period of large sequential and/or mixed epidemics of infection with two subtypes of influenza A virus, H3N2 and H1N1. Of 91 middle-school pupils infected with H3N2 virus shortly before and 82 pupils not previously infected with this subtype, 59% and 91% became infected with H1N1 virus, respectively; this difference was significant. Similar results were obtained at the two primary schools studied. At a high school where epidemics due to the H3N2 and H1N1 subtypes occurred concurrently, the rate of infection of individual pupils with both viruses (2%) was significantly lower than those at the other three schools (21%, 23%, and 31%, respectively), where an epidemic caused by the H3N2 subtype appeared first and was then partially overlapped and succeeded by an epidemic caused by the H1N1 subtype. These findings suggest the existence of cross-subtype protection in humans during sequential and/or concurrent epidemics caused by two viral subtypes.

Reinfection with influenza A virus was studied by measuring hemagglutination-inhibiting antibody responses to infection in paired sera taken from groups of soldiers and students. Among 62 soldiers severely infected during the first wave of the A/Asian/57 (H2N2) pandemic in 1957, 17 were asymptomatically reinfected with the same virus within six months. In the 1962 epidemic the rate increased to 41%. Among reinfected soldiers studied, 68% had an asymptomatic infection; only 10% were severely symptomatic, and they were found to be infected with a virus closely related to A/Asian/57. For H3N2 epidemics, the rate of reinfection was 17% among students studied in 1970 who were reinfected with a virus closely related to the prototype A/Hong Kong/68 (H3N2). Reinfection with an extremely drifted variant of H3N2 was found to be 32% and 69% in two groups of students studied in 1972. Reinfection with a related virus was 32% in another group studied in 1983. Among the students studied who were reinfected with H3N2 viruses, the rates of asymptomatic infection were similar to those of symptomatic infection. The reinfection rates with a virus related to A/USSR/77 (H1N1) were 9.3% and 20% in two groups studied in 1980.

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