Seroprevalence of Tick-Borne Encephalitis Virus in Latvia Using Standard and Subtype-Specific NS1-Based ELISA Assays

<i>Background:</i> Latvia is one of the most endemic tick-borne encephalitis (TBE) countries in Europe. However, assessing TBE seroprevalence and true infection rates has been challenging. Current diagnostics cannot reliably distinguish between immune responses induced by natural infection from those induced by vaccination, especially in TBE endemic countries with recommended immunisation programmes. A recently developed ELISA targeting antibodies against TBEV non-structural protein 1 (NS1) offers improved specificity for natural infection and can differentiate among three clinically relevant TBEV subtypes. <i>Methods:</i> We conducted a cross-sectional TBEV seroprevalence study in the Latvian population during 2019-2022, consisting of two pools: 1020 residents living in different regions of Latvia and 200 random Latvian Biobank blood samples. We used the standard anti-TBEV IgG ELISA (VIDITEST, VIDIA, Czech Republic) for the screening and detection of TBEV (whole virus) IgG antibodies and the newly established research-use anti-TBEV NS1 IgG ELISA for the detection of subtype-specific TBEV NS1 IgG antibodies against three virus subtypes: European, Siberian and Far Eastern. <i>Results:</i> The total TBEV seroprevalence among 1020 residents was 39.7%, representing all age cohorts of the population from all regions of Latvia. In total, 33.4% of the enrolled population were vaccinated against TBE with at least one dose of the TBE vaccine. Among the unvaccinated population, 16.3% had positive TBEV-specific IgG antibodies by standard ELISA. On the contrary, NS1-specific antibodies, indicating past natural infection, were detected in only 4.3% of the overall study population. Subtype differentiation revealed infections from all three major TBEV subtypes present in Latvia. <i>Conclusions:</i> In conclusion, this population-based study highlights a high risk of TBE in Latvia, with substantial seroprevalence even among unvaccinated individuals. The NS1-based ELISA enhances the accuracy of TBE surveillance and offers important clinical utility by facilitating more reliable diagnosis and case classification, regardless of vaccination status.