Tick-Borne Encephalitis and West Nile Virus Antibody Prevalence in the City of Hamburg, a Flavivirus Non-Risk/Low-Risk Area.

In Hamburg, Northern Germany, no locally acquired human Tick-borne encephalitis virus (TBEV) or West Nile Virus (WNV) infections have been recorded as of to date. Since 2018, several WNV infections have been diagnosed in birds/horses. As a majority of TBEV and WNV infections are asymptomatic, diagnoses may be missed. Our objectives were to investigate TBEV- and WNV-antibody prevalence and TBE-protection rates through vaccination in Hamburg. In May 2023, we conducted a TBEV- and WNV-seroprevalence study in 1,321 blood donors at a large donation service; additionally, the percentage of TBE vaccination-induced seroprotection rates were assessed. Laboratory analyses were conducted at the National TBEV Consultant Laboratory. Geographical patterns of TBEV and WNV infections in humans and WNV infections in birds/horses were evaluated. The median age of donors was 43 years (IQR 32-56), 59% were male. TBEV-prevalence was 0.30%. In 24% of blood donors, we detected TBEV-vaccination-induced antibodies; WNV-seroprevalence was 0.30%. No geographical pattern or proximity between TBEV infections was observed. Three human WNV-infections were detected in individuals living in relative proximity. In the non-endemic area Hamburg, a low TBEV and WNV-prevalence could be detected in blood donors. For the first time, TBEV vaccination rates in this non-TBEV-risk area were assessed. Due to the healthy donor effect, vaccination rates may be overestimated. Despite the low seroprevalence of TBEV and WNV in our study, they highlight the need for continuous disease surveillance and local physicians should be aware of possible TBEV and WNV infections, especially in patients with neurological symptoms.