Surveillance of Tick-Borne Encephalitis Virus-Comparison of Vaccination- and Infection-Induced Seroprevalences in Lower Bavaria, Germany.

Tick-borne encephalitis virus (TBEV) is an emerging flavivirus in Europe and Asia, causing severe neurological disease in humans. Recent advances in serological diagnostics, in particular the detection of antibodies against the nonstructural protein 1 (NS1), enable epidemiological research by differentiation of vaccine- and infection-induced antibodies. This assay facilitates precise studies on TBEV seroprevalence. Our study investigates the TBEV seroprevalence in the district of Passau and the surrounding high-risk areas in Lower Bavaria, Germany. A total of 1339 serum samples from voluntary blood donors were examined using a combination of IgG ELISA, NS1-antibody ELISA, and micro-neutralization test (NT). The results showed a TBEV-specific IgG antibody prevalence of 84.8% (1135/1339). Of these, 3.6% (41/1135) were NS1-antibody positive samples, which indicates infection-related antibodies. The vaccination coverage rate was estimated at 81.2% (1094/1339), which is in line with the results from neighboring Austria but exceeds previous vaccination estimates for Bavaria. The study showed a discrepancy between the officially reported incidences (3.7 new cases/100 000 inhabitants annually) and the actual infection rate based on NS1-antibody seroprevalence (180.6 infections/100 000 inhabitants annually). The manifestation index was calculated at 2.1%, confirming that the majority of TBE infections are subclinical or asymptomatic. The results emphasize the usefulness of NS1-antibody-based diagnostics in the accurate assessment of TBEV prevalence and highlight the need for improved surveillance in endemic areas.