Tick-borne encephalitis and the impact of vaccination.

TBE virus is endemic in many parts of Europe and Northern Asia, and in these regions it causes more than 10,000 severe cases of central nervous system disease in humans each year. The virus is primarily transmitted to humans when infected ticks take a blood meal, but infections due to the consumption of unpasteurized milk, primarily from goats, occur in certain regions. Based on genetic analyses, three closely related subtypes can be distinguished and are designated European, Siberian, and Far Eastern subtype according to their primary geographic distribution. Consistent with their close antigenic relationships, immunization studies in animals have revealed a high degree of cross-protection between virus strains belonging to different subtypes. The commercially available vaccines in Europe consist of highly purified inactivated whole TBE virus. Austria is the country with the highest coverage of TBE vaccination (86% of the total population) and this has led to a dramatic reduction in the annual number of clinical cases and proves under field conditions that vaccination is an effective means for the prophylaxis of TBE.