Reinfection with influenza A (H2N2, H3N2, and H1N1) viruses in soldiers and students in Japan.

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.