Reassessing Regional Tick-Borne Encephalitis Endemicity in Poland Through Seroprevalence Analysis in Blood Donors, 2021-2022.

TBEV is a major cause of viral central nervous system infections in Europe, with heterogeneous geographical distribution and substantial underdiagnosis in low-incidence regions. This study aimed to evaluate the validity of regional TBE risk classification in Poland by combining surveillance-based incidence data with serological markers of TBEV exposure. Plasma samples from 5541 blood donors residing in nine regions were tested by anti-TBEV IgG ELISA, followed by confirmatory VNT, IFA, and anti-NS1 IgG ELISA to differentiate infection-induced from vaccine-induced antibodies. Regions were classified based on average TBE incidence registered in surveillance systems in 2015-2019. Overall, 272 (4.9%) donors were positive in TBEV IgG ELISA and 177 (3.2%) also in a confirmatory assay. Seroprevalence expressed by markers consistent with past TBEV infection (anti-NS1 IgG) was estimated at 0.13% (95% CI 0.05-0.26%), ranging from 0.0% to 0.33% by voivodeship, whereas vaccine-induced immunity accounted for the majority of samples with detected specific antibodies (3.1%). Surprisingly, seroprevalence in a highly affected region (0.29%) was at the same level as in one that was less affected (0.33%). Moreover, all except one seropositive donor lived in urban areas. In four out of nine voivodeships, no anti-NS1 TBEV IgG was detected. By utilising precise laboratory algorithms, we demonstrated a much lower seroprevalence that estimated from prior research relying on screening tests. In addition, we confirmed low vaccination coverage. Integrating sero-epidemiological data with surveillance systems may improve risk assessment and inform targeted prevention strategies.