[The characteristics of the epidemic strains of the influenza B viruses that caused the 1990-1991 epidemic in the USSR].

Epidemics of influenza in towns of Russia were found to have been caused by influenza type B virus. Strains similar to B/Yamagata/16/88 were first isolated in the country. The study of antigenic and biological properties showed that the epidemic strains could be divided into 2 groups: similar to B/Victoria/2/87 and to B/Yamagata/16/88, that is, representing two branches of the evolutionary tree. In both groups the viruses were heterogeneous both in antigenic and biological properties.