[A live vaccine against tick-borne encephalitis: the principles of selecting the vaccinal strains].

The comparative, semiquantitative, pathomorphological study of the neurovirulence of clones of Elantsev virus and Langat virus TP-21 for intracerebrally infected monkeys has been carried out. The study has revealed that the viruses may be differentiated by their neurovirulence for primates according to the average statistical data on the degree of pathomorphological changes in the central nervous system, but not to maximum lesions in cerebral structures. The level of neurovirulence of yellow fever virus 17D was formerly considered to be the highest admissible limit of residual neurovirulence of encephalitogenic viruses (flaviviruses). According to our data, Elantsev virus, used for the immunization of humans and known to have caused some cases of encephalitis, is similar to yellow fever virus with respect to its neurovirulence for primates: therefore, a candidate strain intended for the preparation of tick-borne encephalitis (TBE) vaccine must be significantly less neurovirulent. The neurovirulence of clones isolated from Langat virus TP-21 has proved to be essentially lower than that of Elantsev virus clones. Langat virus TP-21 is a promising source of clones suitable for use as candidates for live TBE vaccine. Search for vaccine strains by testing their neurovirulence in experiments on several strains of mice and their hybrids, on hamsters and on immunosuppressed animals is methodologically groundless. The adequate evaluation of the level of residual neurovirulence of viruses to be used as candidates for live TBE vaccine can be made only on monkeys.