[Attenuated recombinant influenza type B virus obtained during crossing of virus B/Ann Arbor/2/86 with the cold-adapted strain B/Leningrad/14/17/55].

Crossing the cold-adapted B/Leningrad/14/17/55 strain with the temperature-sensitive virulent B/Ann Arbor/2/86 strain yielded a recombinant B/14/5/1 which, by the antigenic specificity of hemagglutinin and neuraminidase, corresponded to the B/Ann Arbor/2/86 strain but, like the attenuated donor, had the cold-adapter characteristics. The B/14/5/1 recombinant inherited the genes coding for proteins PB2, PB1, PA, NP, and M from the attenuated master strain and the genes coding for hemagglutinin, neuraminidase, and proteins NS from the virulent master strain. This strain was nonreactive for adults and for children with the initial anti-hemagglutinin antibody titre less than or equal to 1:20 (the reactogenic index being 1 and 0.9% respectively) and was moderately antigenic inducing a 4-fold or more rise of anti-hemagglutinins in the blood of 48.8% of seronegative adults and in 46.6% of seronegative children of 3 to 15 years of age. The highest indices of seroconversions (60%) were recorded in a group of preschool children.