Ability of inactivated vaccines based on far-eastern tick-borne encephalitis virus strains to induce humoral immune response in originally seropositive and seronegative recipients.

Tick-borne encephalitis (TBE) remains one of the major public health concerns in northern Eurasia, and its' area is expanding. TBE virus (TBEV) includes three subtypes and several monophyletic groups, cocirculating in Russia. Five inactivated vaccines are used for TBE prophylaxis. The rising number of people subjected to vaccination brings up the issue of the impact of individual recipient characteristics on vaccination efficacy. The present work studies correlations among the vaccination scheme, sex, age, body mass index (BMI), chronic diseases, postvaccinal reaction, pre-existing anti-TBEV antibodies, and postvaccinal humoral immunity development. Sera were collected during clinical trials in the TBEV Siberian subtype endemic area. Adult recipients were vaccinated with Tick-E-Vac and EnceVir vaccines based on Far-Eastern TBEV strains. Vaccine ability to induce humoral immunity in different categories of recipients was estimated by seroconversion rates and the percentage of recipients with high neutralizing antibody titers (≥1:500). High immunogenicity of vaccines based on Far-Eastern TBEV strains in the TBEV Siberian subtype endemic area in all groups of recipients was demonstrated. Impact of pre-existing contact with the virus and high BMI on humoral immune response development 14 days after the first immunization was evidenced. Nevertheless, the difference was significantly less pronounced 30 days after the first vaccination and undetectable after the second one.

Comparison of the Immunogenicity and Safety of Two Pediatric TBE Vaccines Based on the Far Eastern and European Virus Subtypes.

Up to 10,000 cases of tick-borne encephalitis are registered annually, 20% of which occur in children under 17 years of age. A comparison of the immunogenicity and safety between a new pediatric Tick-E-Vac vaccine based on the TBEV strain Sofjin and FSME-IMMUN Junior vaccine was performed in the Sverdlovsk region. The vaccine strains differ from strains of the Siberian subtype of TBEV that dominates in the region. The study was performed on 163 children aged 1 to 15, who received one of the vaccines according to either a conventional or rapid vaccination schedule. Immunogenicity was assessed based on the seroprotection rates and titers of virus-neutralizing antibodies. There were no significant differences in either the immunogenicity or reactogenicity of the pediatric vaccines based on strains of the Far Eastern or European subtypes of TBEV. Under both vaccination schedules, 30 days after the second injection, seroprotection rates were 100% for Tick-E-Vac and greater than 95% for FSME-IMMUN Junior, while the geometric mean titer of TBEV-neutralizing antibodies was at least 2,4 log10 (1 : 250) for either vaccine. Fourteen days after the second injection according to the rapid schedule, seroprotection rates were significantly lower, ranging from 50% to 63% regardless of the vaccine used. The observed adverse reactions were mild or moderate for both vaccines under both vaccination schedules, with total adverse event rates of less than 25%. Reactogenicity was not associated with the gender or age of the recipients. There were no statistically significant differences in the incidence of adverse reactions between the group of subjects who were baseline seronegative or seropositive. However, 14 days after the second vaccine injection according to the rapid schedule, a statistically significant difference in nAbs titers was identified between groups of children with and without reported reactions.