Further genetic evidence for maintenance of early Hong Kong-like influenza A(H3N2) strains in swine until 1976

The oligonucleotide map of the whole RNA of A/swine/Hong Kong/3/76 (H3N2) virus showed a relationship to, but considerable differences from, the maps of human isolates of influenza A(H3N2) with which this isolate antigenically cross-reacted. Comparison of the individual oligonucleotide spots of whole virus RNAs confirmed previous results of antigenic analysis that A/swine/Hong Kong/3/76 virus was more similar to an early A/Hong Kong/68 virus than to later H3N2 viruses which circulated in man during 1976. On the other hand, another swine isolate, A/swine/Hong Kong/4/76, showed a quite similar oligonucleotide map to that of the contemporary prevalent A/Victoria/75-like viruses to which this strain was reported to be antigenically similar. Comparison of oligonucleotide maps of individual RNA segments indicated that all genes of the A/swine/Hong Kong/3/76 virus were derived from a human H3N2 virus. The findings provide biochemical evidence that A/swine/Hong Kong/3/76 virus represents a 1968 Hong Kong-like virus that underwent genetic mutation without extensive change of its antigenicity during maintenance in the swine population.