Studies on influenza virus-virulence in recombinants between epidemic and vaccine strains

: Influenza virus recombinants between epidemic strains A/Brazil/11/78 (H1N1), A/USSR/382/78 (H3N2) and vaccine strains A/Leningrad/9/46 (H1N1), A/Victoria/35/72/50 (H3N2) have been tested for virulence for humans and albino mice; their genome structure has also been determined. It has been shown that after the replacement of surface antigens of A/Leningrad/9/46 (H1N1) strain by surface antigens of A/Brazil/11/78 (H1N1) or A/USSR/382/78 (H3N2), strains, the virus becomes totally nonpathogenic for mice whereas its virulence for humans is enhanced. The combination in recombinant X/28 (H1N1) of haemagglutinin and neuraminidase of A/Brazil/11/78 (H1N1) virus and othercomponents of A/Leningrad/9/46 virus determines its high affinity to the epithelium of the upper respiratory tract of humans, as well as its marked virulence for seronegative volunteers. Genetic mechanisms of influenza virus virulence and the involvement of surface proteins in its specific manifestations are discussed. It has been shown that pathogenic properties and the affinity of the virus to particular tissues are determined by different genes and their reasortment can result in the appearance of essentially new properties in recombinants.