Tick-borne encephalitis virus: reference strain Sofjin and problem of its authenticity

S. Y. Kovalev, T. A. Mukhacheva, V. S. Kokorev, I. V. Belyaeva

The Sofjin strain is one of the first isolates of tick-borne encephalitis virus and, due to its wide distribution in virus collections, it has become the reference strain. Until now, GenBank has recorded several sequences associated with the Sofjin strain that have significant differences between each other. We have sequenced the complete genome of the Sofjin strain from a virus collection and a genome fragment of the two vaccine Sofjin strains. According to phylogenetic analysis, we concluded that the GenBank sequences belong to three independent groups of Sofjin strains of the Far Eastern subtype. Their genetic differences are not a result of microevolution associated with numerous passages. Retrospective analysis of the peculiarities of origin and distribution for each of these groups showed the authenticity of one of them. For the first time, we have determined a complete genome sequence of the authentic reference TBEV strain Sofjin. Two other groups of strains named Sofjin were probably the result of cross-contamination or laboratory error. The high probability of contamination requires the introduction of a new standard for virological laboratories, the key point of which is the obligatory genetic identification of all collection strains.