Facets of virulence for ferrets of 16 recombinant clones of two parent viruses A/Finland/4/74 (H3N2) and A/Okuda/57 (H2N2) were determined and viewed in relation to their genetic composition. Of the five pairs of recombinant clones with RNA segments of the same parental origin, differences in facets of virulence were detected between members of two of the pairs. One pair differed in ability to produce fever, and another pair in ability to infect the lower respiratory tract. Subsequent analyses indicated slight differences in two genes of the first pair of viruses. Of all the recombinant clones examined the three that derived their surface antigens from the attenuated parent (A/Okuda) were less virulent than the 13 which possessed the surface antigens of the virulent parent (A/Finland). The virulence of the latter clones tended to decrease as the number of RNA segments that were derived from A/Okuda increased.


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