Tikkala et al.
Tick-borne encephalitis in the Åland Islands, Finland: incidence and disease course since the implementation of a general TBE vaccination programme. Emerg Microbes Infect. Published online March 4, 2026. doi:10.1080/22221751.2640705
The Åland Islands, located in the Baltic Sea off the southwestern coast of Finland, have a population of approximately 31,000 and constitute an endemic area for tick-borne encephalitis (TBE). TBE, locally known as Kumlinge disease, was first recognized in the Åland archipelago in the 1940s, and the virus was first isolated there in 1959. Until 2004, TBE cases from Åland accounted for the majority of all reported cases in Finland, which led to the introduction of a general vaccination program in Åland in 2006. Since then, unlike in mainland Finland, no increase in the number of TBE cases has been observed in Åland; however, the incidence has remained persistently high, ranging from 7 to 76 cases per 100,000 persons per year.
This study was conducted to describe the incidence, clinical features, disease severity, diagnostic challenges, and outcomes of TBE in Åland from 2006 to 2023. During the study period, 145 patients were diagnosed with TBE in Åland, corresponding to an incidence of 29 per 100,000 person-years. Of these patients, 9.7% were younger than 18 years. Overall, 49.7% were unvaccinated, 31.7% had received an incomplete vaccination series, and 13.8% were fully vaccinated.
Among the patients, 39.3% presented with meningitis, classified as mild disease, while 60.7% presented with meningoencephalitis, of whom 51.0% were classified as having moderate disease and 9.7% as having severe disease. Intensive care was required in 9.0% of cases, and two fatal cases occurred (1.4%). A monophasic course of disease was observed in 62.8% of patients. Increasing age was significantly associated with greater disease severity. Incomplete vaccination and immunosuppression were also significantly associated with severe disease. In addition, in 18% of patients with complete or incomplete vaccination, no IgM antibodies were detectable in the initial serum samples, compared with 7% in the unvaccinated group.
Despite the high morbidity associated with TBE, 86% of patients were considered recovered two years after the acute phase. Among the 119 patients included in follow-up, 16% had persistent symptoms at the end of the two-year follow-up period.
In conclusion, the epidemiology, symptoms, and clinical signs of TBE in Åland have not changed significantly over time, despite high vaccination coverage of more than 70% following implementation of the general vaccination program. The relatively high proportion of breakthrough infections was an unexpected finding and is discussed in detail.