Cross-subtype protection in humans during sequential, overlapping, and/or concurrent epidemics caused by H3N2 and H1N1 influenza viruses.

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.