Tick-borne encephalitis virus IgG antibody surveillance: vaccination- and infection-induced seroprevalences, south-western Germany, 2021

BACKGROUND: The exact epidemiology of tick-borne encephalitis virus (TBEV) infections is unknown because many TBEV infections have an influenza-like or asymptomatic course. Surveillance data are based on patients with any (predominantly neurological) symptoms that prompted diagnostic testing. Infection- and vaccine-induced antibodies against TBEV can be distinguished using an NS1 IgG ELISA. AIM: In a seroprevalence study we aimed to investigate TBEV antibody prevalence, incidences, manifestation indices and potential protection rates in a highly endemic district in south-western Germany. METHODS: We analysed 2,220 samples from healthy blood donors collected between May and September 2021. The reported number of TBEV infections was provided on a sub-district level by the local public health authorities. Blood samples were first screened using a TBEV IgG ELISA. In a second step, all positive samples were further analysed with a recently established NS1 IgG ELISA. The presence of specific antibodies against TBEV (excluding cross-reacting antibodies against other flaviviruses) was confirmed by testing screening-positive samples with a microneutralisation assay. RESULTS: Of 2,220 included samples, 1,257 (57%) tested positive by TBEV IgG ELISA and 125 tested positive for infection-induced TBEV NS1 antibodies, resulting in a TBEV NS1 IgG seroprevalence at 5.6% in our population. The yearly incidence based on the NS1 ELISA findings resulted in 283 cases per 100,000 inhabitants. CONCLUSION: Using the TBEV NS1 IgG assay, we confirmed a manifestation index of ca 2% and a high incidence of predominantly silent TBEV infections (> 250/100,000/year), which exceeds the incidence of notified cases (4.7/100,000/year) considerably.