Tick-Borne Encephalitis Vaccine Effectiveness and Impact in Slovenia, 2019-2024

Slovenia has a high tick-borne encephalitis (TBE) incidence and TBE vaccination is recommended for all residents ≥1 year-of-age. Despite widespread TBE vaccine use in Slovenia, TBE vaccine effectiveness (VE) has not been reported for Slovenia. We defined individuals who received ≥3 TBE vaccine doses in accordance with the recommended vaccination schedule as fully vaccinated. Using the screening method, TBE VE was determined by comparing the proportions of TBE cases and survey respondents who were fully vaccinated. TBE incidence and VE were used to estimate the number of TBE cases prevented by TBE vaccination. Of 637 surveillance-reported TBE cases in 2019-2024, 332 (52.1%) had a known vaccination history. Of these 332 TBE cases, 322 (97.0%) were unvaccinated and 10 (3.0%) received ≥1 doses of a TBE vaccine. Among 25,242 persons surveyed with known TBE vaccination history, 57.9% were unvaccinated. VE of ≥3 TBE doses for the prevention of TBE was 93.4% (84.0-97.3%). TBE vaccination prevented 426 TBE cases in 2019-2024. If the entire population was vaccinated against TBE, vaccination would have prevented 994 TBE cases in 2019-2024. TBE vaccination in Slovenia was highly effective, preventing hundreds of TBE cases a year. Most of the Slovenian population, however, is not vaccinated against TBE. To prevent additional TBE cases in Slovenia, further efforts are needed to enhance TBE vaccine uptake.