The passage of newly isolated, filamentous Asian (A2) influenza viruses in the presence of non-infective PR8 (A) virus results in the rapid emergence of virus of Asian (A2) antigenicity but PRS-like growth capacity and spherical morphology.

Evidence is presented that this effect results from genetic interaction of the infective Asian and non-infective PR8 viruses rather than from spontaneous change of the Asian strain.

It is concluded that influenza viral morphology, growth rate and growth capacity are associated genetic traits which distinguish unadapted from adapted strains, and which are transferable by recombination.

A pragmatic consequence of these experiments is the fact that conditions have been defined for the rapid adaptation of early passage influenza virus isolates to the chick embryo allantoic sac. Such adaptation is attended by an increase in viral yield which has obvious implications for vaccine production during future epidemics with new antigenic types.

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