148 of 157 invited adult subjects who had participated in previous studies were enrolled in this extension study for evaluation of immunogenicity and safety of the second TBE booster immunization. All subjects had been previously immunized in studies with Chiron's formerly marketed TBE vaccine (containing polygeline as the stabilizer) according to the rapid vaccination schedule (i.e. primary immunization on days 0, 7, 21 and first booster immunization at month 15). All subjects were administered the second booster with Chiron's new TBE vaccine, which is free of protein-derived stabilizers, 36 months after the first booster vaccination applied at study month 15. Blood samples were taken prior to booster and 1 month later. In 145 out of 148 subjects, blood samples suitable for measurements of TBE antibodies (ELISA assay) were provided. Prior to second booster immunization with Chiron's new TBE vaccine, TBE antibodies (GMTs) had remained at a high level and were far above the detection limit of the used ELISA test. All subjects were still seropositive prior to the second booster immunization. The second booster immunization resulted in a further increase of TBE antibodies. The booster vaccination with Chiron's new TBE vaccine was well tolerated by all the vaccinees. Neither febrile post-immunization reactions nor unexpected adverse events or serious adverse events were reported. To summarize, these data clearly show that the TBE vaccination with this new TBE vaccine can be used safely to boost subjects pre-immunized with the former TBE vaccine formulation. Long-lasting immunity following this second TBE booster immunization can be concluded.