Tick-borne Encephalitis (TBE) can be prevented by vaccine. Vaccine failure, defined as a case
of TBE regardless of previous vaccination, has been described and seems to be more
predominant with increasing age, suggesting a less effective immune response following with
increasing age. In fact previous studies has shown a reduced antibody response in elderly
individuals compared to younger when vaccinated against TBE. As a result, in Sweden, an extra
vaccine dose has been recommended during the primary vaccine schedule to individuals > 50
years of age. This alternative vaccine schedule has not been tested. The investigator aim to
test if an extra vaccine dose in the primary vaccine schedule for those > 50 years of age
improves the immune response and offers a corresponding immunity to younger individuals
following TBE vaccination.