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 5,541 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 from 2015–2019. Overall, anti-TBEV IgG screening reactivity was detected in 4.9% of donors, with significant regional variation (p < 0.001). The highest seroprevalence was observed in highly affected regions; however, unexpectedly elevated seroprevalence was also detected in regions classified as low affected. Markers consistent with TBEV infection (anti-NS1 IgG) were identified in only 2.6% of donors, whereas vaccine-induced immunity accounted for the majority of seropositive results. Male sex was independently associated with higher odds of seropositivity. Our findings suggest that passive surveillance data alone may insufficiently capture population-level exposure to TBEV, particularly in regions considered non-endemic. Integrating sero-epidemiological data with surveillance systems may improve risk assessment and inform targeted prevention strategies.