Morgardt M, et al.
A rare case of breakthrough tick-borne encephalitis in early adolescence after vaccination with four doses of TBE vaccine. New Microbes New Infect. 2026; in press. doi:10.1016/j.nmni.2026.101720
Various studies have demonstrated that tick-borne encephalitis (TBE) vaccination is highly effective, with breakthrough infections occurring in fewer than 10% of individuals after completion of the primary vaccination series (three doses).
A case of a breakthrough TBE infection in a Swedish adolescent has been reported. The patient had received two doses of the TBE vaccine seven years before disease onset, a third dose three years before, and a fourth booster dose three months before symptom onset. The illness began with fever and headache (day 0). On day 5, the patient was admitted to the hospital due to neck stiffness and photophobia. Empirical treatment with antivirals, antibiotics, and betamethasone was initiated.
Initial serological testing showed TBE-specific IgG positivity and IgM negativity in serum, while cerebrospinal fluid (CSF) was weakly TBE-IgM positive, which was initially interpreted as a nonspecific reaction. Subsequent analyses revealed high IgM and IgG titers against the TBE virus non-structural protein 1 (NS1).
After three days, the patient’s clinical condition deteriorated. On day 11, serum testing demonstrated TBE-IgM positivity with rising TBE-IgG levels, confirming the diagnosis of breakthrough TBE infection. Extensive microbiological investigations for alternative infectious causes, as well as testing for autoimmune antibodies in CSF, were negative. On day 14, brain MRI revealed pathological abnormalities.
Following nearly three months of hospitalization, the patient was discharged home on day 87. Although gradual clinical improvement was observed over subsequent months, persistent neurological sequelae remained. Even two years after disease onset, the patient continued to experience cognitive impairments, including deficits in social communication, memory impairment, and difficulties with daily-life organization.
Despite extensive evaluation and discussion, the underlying reason for the unusually severe disease course remains unknown.