# Infection and Vaccination-Induced Tick-Borne Encephalitis Virus IgG Antibody Prevalence in the Austrian Federal State of Upper Austria, a High-Risk Region for TBEV

## TL;DR

This study examines the prevalence of tick-borne encephalitis virus (TBEV) antibodies in Upper Austria, showing high vaccination rates and a significant infection risk for the unvaccinated.

## Contribution

The study introduces a method to distinguish between vaccine- and infection-induced TBEV antibodies, enabling more accurate sero-epidemiological analysis.

## Key findings

- 87.0% of blood donors had anti-TBEV-IgG from vaccination.
- Unvaccinated males had a higher infection rate (13.3%) than females (9.9%).
- The manifestation index for severe TBEV infection was 2.8%.

## Abstract

Background/Objectives: Tick-borne encephalitis (TBE) is the most important tick-borne viral central nervous system (CNS) infection in Europe and Asia. Since the introduction of a vaccine in Austria in the late 1970s, sero-epidemiological studies on the true incidence of tick-borne encephalitis virus (TBEV) infection in the population have been difficult, because it was not possible to distinguish between vaccine- and infection-induced antibodies. The goal of our study has been to analyze the sero-epidemiology of TBEV infections, vaccination protection rate, and manifestation index of the disease in the region of interest. Methods: Applying a newly developed anti-TBEV-NS1-IgG assay and the neutralization test, the protection and infection rates in blood donors of the Austrian Federal State of Upper Austria.It is one of the first areas in Austria, where the TBEV vaccine had been rolled out and broadly used. Samples from blood donors of all districts of the Federal State of Upper Austria were screened for anti-TBEV-IgG. Positive sera were differentiated for infection- and vaccine-induced antibodies. The results were matched with donor age, gender, and geographical origin. Results: 2162 samples were analyzed. A total of 87.0% of the blood donors tested showed anti-TBEV-IgG related to past TBEV vaccination. Within the unvaccinated group, a total of 13.3% of male and 9.9% of female blood donors exhibited anti-TBEV-NS1-IgG, indicating a past TBEV infection. The anti-TBE-NS1-IgG seroprevalence was determined at 74/100,000 for the whole population and at 594/100,000 in the non-vaccinated population. The manifestation index (MI) was calculated at 2.8%. The MI is defined as the probability or percentage of infected individuals who develop clinical symptoms of a disease. Conclusions: Our data provide evidence of a continuing high risk of TBEV infection in the Federal state of Upper Austria. The non-vaccinated population has an eightfold higher infection prevalence compared to the whole population. The MI of TBEV for severe infection seems lower as detailed in previous reports.