Tick-Borne Encephalitis Virus Surveillance-Vaccination- and Infection-Induced Seroprevalences in Western Austria 2024.

Reported tick-borne-encephalitis (TBE) cases have been increasing in Western Austria, but no data are available on vaccination- and infection-specific seroprevalence. This study aimed to estimate current TBEV-seroprevalence in the region and inform prevention programs by comparing anti-NS1-based-incidence rates with reported case numbers and vaccination coverage. Between December 2023 and February 2024, serum samples from 4619 blood donors in Western Austria were collected and analyzed using TBEV- and WNV-IgG-ELISA assays. Seropositive samples were tested with a TBEV-anti-NS1-IgG-ELISA to distinguish infections from vaccinations. Borderline samples were retested with serum neutralization and triple-NS1-assays. The overall anti-TBEV-IgG-seroprevalence was 80.1% (95%CI 78.9-81.3); 2.7% (95%CI 2.3-3.2) of donors tested positive for anti-TBEV-NS1 IgG antibodies, indicating previous infection. The notified incidence rate in Western Austria was 2.7/100 000/year, compared to 136.2/100 000/year based on anti-TBEV-NS1-seropositive donors, denoting a substantial number of unreported cases (mean manifestation index 1.9%). The number of donors with TBEV-infections varied considerably by district, highlighting potential hotspots for TBEV-infections. The high anti-NS1-based, estimated annual TBE incidence rates show significant differences between districts, highlighting the need for targeted prevention programs. The high rate of undiagnosed TBE cases further suggests that estimated anti-NS1-based incidence rates should be considered when defining high-risk areas.