Protocol amendment of October 2006: The study has been prolonged for children and adolescents who still showed highly positive TBE virus antibody concentrations at approximately 3 years after the third vaccination and therefore did not receive a booster vaccination at this time point. Further follow-up of TBE antibody persistence is now included for these subjects at 46 and 58 months after the third vaccination (in Study 209), as well as a booster vaccination offered at either 48 or 60 months after the third vaccination (in Study 209), depending on individual TBE antibody levels.