Orlova et al.
The immune responses after vaccination in recipients against different subtypes of tick-borne encephalitis virus (Flaviviridae: Orthoflavivirus) Probl. Virol. 2025, 70: 559-569, doi.org/10.36233/0507-4088-348
There are three antigenic subtypes of the TBE virus: the European, the Siberian and the Far Eastern. In addition, the Baikal and the Himalayan subtypes have been identified. The Sib subtype is the most extensively distributed subtype and circulates in Western and Eastern Siberia, European Russia, the Baltic countries and Finland.
However, the three available TBE vaccines produced in Russia are not based on a Sib strain, but on a FE strain (the Sofyin strain: TBE-Moscow and Klesch-E-Vak or strain 205: Encevir). These TBE vaccines are the most widely used in Russia, while no current TBE vaccine is based on a Sib subtype (FSME-IMMUN and Encepur are based on EU strains).
A Russian team analyzed if antibodies produced after vaccination with FE-based TBE vaccines immunologically react with the FE strain Sofyin, the Sib strain Vasilchenko, the EU strain Absettarov, and the Sib strain (Zausev linage) Ekb_1887_1. Sera were collected from individuals aged 10-70 years of age and living in the Sverdlovsk region and who had received 1 to 18 vaccine doses. The IgG antibody titers measured by a commercial ELISA based on a FE vial antigen ranged from 1:10 to 1: 2500.
Individuals vaccinated at least three times showed sufficient heterologous neutralizing capacity. No clear correlation between the number of vaccinations and antibody levels was identified in this study. Low-titer sera did, however, not reach the minimum protective level of 1:10 against Sib and/or EU strains. These results indicate that there may exist a limited cross-protection of the FE based TBE vaccines.
The authors concluded that it may be worth to develop a TBE vaccine based on a Sib subtype, as it is the most widespread subtype in Russia. Another promising approach could be the development of a combination vaccine based on several subtypes.