Tick-borne encephalitis vaccine effectiveness and public health impact in the Baltic countries of Estonia, Latvia, and Lithuania, 2019-2023.

Objectives: Tick-borne encephalitis (TBE), a potentially life-threatening disease caused by the TBE virus, is an emerging European public health threat. TBE is endemic nationwide in Estonia, Latvia, and Lithuania, which have the highest TBE incidence in Europe. TBE vaccination is recommended for all Baltic residents ≥1 year of age, but estimates of the public health impact of TBE vaccination are limited. Methods: TBE vaccination histories were determined for surveillance-reported TBE cases and for respondents from general population surveys conducted in 2019-2023. Vaccine effectiveness (VE) of ≥3 TBE doses administered in accordance with the vaccination schedule (i.e., fully vaccinated) was determined by comparing the proportion of TBE cases who were fully vaccinated (PCV) with the proportion of survey respondents fully vaccinated (PPV), using the screening method formula: VE = 1-[PCV/(1-PCV)]/[PPV/(1-PPV)]. Results: There were 4,361 surveillance-reported TBE cases in the Baltic in 2019-2023; 3,806 (87.3%) were hospitalized, and 30 (0.8%) hospitalized cases died. Of 4,138 surveillance-reported TBE cases with a known TBE vaccination history, 98.3% were unvaccinated, 1.4% partially vaccinated, and 0.3% fully vaccinated. There were 89,656 participants in the general population surveys; of the 80,970 with a known TBE vaccination history, 50.1% were unvaccinated, 29.8% partially vaccinated, and 20.1% fully vaccinated. VE against TBE was 97.4% (93.0-99.0) in Estonia, 99.0% (97.7-99.5) in Latvia, and 99.6% (95% CI 98.8-99.9) in Lithuania. TBE vaccination averted an estimated 3,520 TBE cases in the Baltic in 2019-2023. Conclusions: TBE vaccination in Estonia, Latvia, and Lithuania was highly effective in preventing TBE. To prevent life-threatening TBE, TBE vaccine uptake and compliance with vaccination recommendations should be increased in the Baltic countries.