The effect of the TBE vaccination program in the Åland Islands

The Åland Islands included tick-borne encephalitis (TBE) vaccination in the general vaccination program in 2006. Investigate the effect of the vaccination on the number of reported TBE cases and the TBEV IgG seroprevalence in blood donors in the Åland Islands. We used reported data on TBE cases (1995–2018) and sera collected from blood donors in 1995 ( n = 300) and 2018 ( n = 300). Samples were analyzed by a Luminex-based method that can differentiate antibodies induced by a TBE virus (TBEV) infection from those produced after TBE vaccination. A weak negative trend but no significant relationship between the number of reported TBE cases and year was observed. Of the blood donors, 3.3 % and 7.0 % tested positive for a previous TBEV infection in 1995 and 2018, respectively. There was no significant difference between the blood donor cohorts regarding the number of TBEV-infected and non-infected individuals. The proportion of TBE vaccinated blood donors increased from 2.7 % in 1995 to 81.0 % in 2018. The proportion of previously TBEV-infected unvaccinated blood donors increased from 3.4 % in 1995 to 36.8 % in 2018. The estimated number of unvaccinated individuals decreased 3.8-fold from 1995 to 2018. The rate of TBE cases in the estimated unvaccinated population increased 3.9-fold between the years 1995 and 2018. The risk of being infected by TBEV tended to be higher in 2018, reduced for men, and to increase with age. The strong increase in seroprevalence of anti-NS1 antibodies and increase of TBE cases in the estimated unvaccinated population seen in this study suggest that the low number of TBE cases in the Åland Islands is explained by the high vaccination coverage, suggesting a positive effect of the free TBE vaccination on public health in the Åland Islands. • TBE vaccination was included in the Ålandic general vaccination program in 2006. • High TBE vaccination coverage (81 %) among Ålandic blood donors in 2018. • High increase of TBE virus-infected in the estimated unvaccinated population. • Stable TBE incidence during 1995–2018 despite increase in surrounding areas. • Suggested positive effect of the free TBE vaccination on public health in the Ålandic Islands.