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Influenza H1N1 viruses isolated in Japan in 1978-1979 were compared with A/USSR/92/77 and H1N1 viruses isolated in North and South America in 1978 by oligonucleotide mapping of viral RNA. At least four types of H1N1 viruses, distinguished by oligonucleotide pattern, were involved in influenza outbreaks during 1978-1979 in Japan. The first type was prevalent throughout the country from January to March 1978. The second and third types caused sporadic outbreaks during the 1978-1979 winter. The second type was closely related to an H1N1 strain of 1953, A/Omachi/1/53; the third type appeared to be reintroduced from outside of Japan. The fourth type was prevalent from January 1979 and persisted to September 1979. Its origin was a virus closely related to A/California/45/78 virus, which appeared in the United States in late 1978. The oligonucleotide pattern of A/California/45/78 viral RNA was markedly different from that of other 1977-1978 H1N1 viruses; the major change occurred in the polymerase and nucleoprotein genes.

Antigenic analysis of influenza A(H1N1) virus isolates by hemagglutination inhibition reactions with postinfection ferret sera and mouse monoclonal antibodies showed that, although the predominant virus in 1978 to 1979 had an A/Brazil/11/78-like hemagglutinin, several new antigenic variants could be detected which differed from those identified in 1977 to 1978. Most of the variants showed very minor changes and did not achieve epidemiological significance. Genome analysis by ribonucleic acid-ribonucleic acid hybridization, and supported by oligonucleotide mapping, indicated that nonrecombinant H1N1 viruses cocirculated in the world with recombinant H1N1 viruses, identified in 1978 to 1979. The results of our antigenic analysis and genomic analysis, in conjunction with genomic analysis by other authors, showed that recombinants with A/USSR/90/77-like and A/Brazil/11/78-like hemagglutinin had different genome compositions, indicating that H3N2 virus recombined separately with these different antigenic variants. Prevalence of the nonrecombinant and recombinant H1N1 viruses varied between regions of the world, and influenza A(H1N1) virus with a genotype almost identical to that of nonrecombinant A/Brazil/11/78 virus reappeared as a cause of local outbreaks in the United States in 1980 despite the predominance of recombinant H1N1 strains in the preceding winter.