Hoffman et al.
The Effect of the Tick-Borne Encephalitis Vaccination Program in the Åland Islands Vaccine X. 2025;27:100727. doi:10.1016/j.jvacx.2025.100727
The Åland Islands, an autonomous territory under Finnish sovereignty in the Baltic Sea, have approximately 30,000 inhabitants. This region is highly endemic for the tick-borne encephalitis (TBE) virus. Since 2006, TBE vaccination has been included in the childhood immunization program free of charge. Approximately 70% of inhabitants aged 7 years and older have been fully vaccinated.
A study was conducted to investigate the effect of including TBE vaccination in the general immunization program on the number of reported TBE cases during 1995–2018, as well as on the IgG seroprevalence in blood donors in 1995 and 2018.
Between 1995 and 2018, a total of 231 TBE cases were reported in the Åland Islands (annual range, 2–26), corresponding to an incidence of 7.9–99.5 (mean, 36.7) cases per 100,000 inhabitants. During the 5-year period from 2014 to 2018, the average incidence decreased to 24.1 (a 29.5% reduction), whereas incidence increased in mainland Finland and Sweden.
Using a test assay capable of distinguishing between antibodies directed against the whole virus (indicating vaccination) and the nonstructural protein NS1 (indicating infection), the proportion of TBE-vaccinated blood donors increased markedly from 2.7% in 1995 to 81.0% in 2018. The estimated number of vaccinated individuals aged 15–65 years or older increased from approximately 400 in 1995 to 19,500 in 2018—a 48.8-fold increase—indicating fewer individuals at risk of TBE virus infection. However, the incidence of TBE among the unvaccinated population increased 3.9-fold between 1995 and 2018, accompanied by a marked increase in NS1 antibody positivity in this group.
The authors concluded that the low number of TBE cases in the Åland Islands can be explained by the high vaccination coverage. The persistence of TBE cases despite this coverage may reflect increased exposure risk to TBE virus, as supported by the continued observation of 40% TBE antibody seroprevalence in dogs in the region. Regular TBE booster doses are expected to further improve immunity in the vaccinated population.