Attenuated influenza B virus recombinants obtained by crossing of B/England/2608/76 virus with a cold-adapted B/Leningrad/14/17/55 strain.

: Crossing of an attenuated influenza B virus strain (B/Leningrad/14/17/55) passaged at a low temperature with a virulent influenza B virus strain (B/England/2608/76) yielded recombinants similar in the antigenic specificity of their haemagglutinin (HA) and neuraminidase (NA) to B/England/2608/76 strain, but possessing an RCT37.5 marker alike to the attenuated donor. Analysis of the genome composition of 2 recombinants has shown that they inherited genes coding for P (1, 2, 3) and M (7) proteins from the attenuated parent, but genes coding for HA (4), NA (6), NP (5) and NS (8) proteins from the virulent parent. All recombinants proved to be areactogenic for adult volunteers with no pre-existing antibody to the corresponding HA (less than or equal to 8); however, they had a reduced immunogenicity as compared to parent viruses.