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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.

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SUMMARYIn 1974 an epizootic occurred among budgerigar flocks in Kunitachi, Tokyo, and a causative agent which possessed haemagglutinating, neuraminidase, and haemolytic activities was isolated from the lung of a dead budgerigar. This agent was IOO to 3o0 nm in diameter and pleomorphic. The width of the ribonucleoprotein was estimated to be about 2o nm. These results indicated that the virus, designated Kunitachi virus, was a member of the paramyxovirus group. The virus contained in the amniotic fluid from infected embryonated hen's eggs, however, at times displayed no haemagglutinating activity with different erythrocytes and complete haemagglutination could only be detected in purified preparations. The Kunitachi viruses including three strains recently isolated from the same host were found to be serologically distinct from the known paramyxovirus strains and appeared to constitute a new subtype of avian paramyxovirus.