Tick-borne encephalitis virus (TBEV) is spreading geographically, and new risk areas are expected throughout Denmark. In this case report, we describe the first Danish case of vaccine breakthrough tick-borne encephalitis (TBE) in a 76-year old female suffering from severe symptoms with need of intensive-care therapy. We want to draw attention to TBE as a differential diagnosis in all undiagnosed patients with symptoms of viral encephalitis, regardless of travel history and vaccine status. TBEV can cause severe disease, especially in the elderly. Patients with vaccine breakthrough infection may develop a more severe disease than non-vaccinated.