Immunogenicity and safety of a booster vaccination against tick-borne encephalitis more than 3 years following the last immunisation.

This study investigated the immune response and safety in 430 adults, when boosted more than 3 years after primary or booster TBE immunisation as measured by neutralization test (NT) and ELISA. Tested by NT, the post-booster day 21 geometric mean titer (GMT) was 331 and 142 for the 18-49 and > or =50 years old, respectively. The post-/pre-booster geometric mean titer ratio (GMR) was 2.29 for the 18-49 years old and 3.21 for the > or =50 years old. An at least four-fold increase of neutralizing TBE antibodies was observed in only 26 and 38% of subjects aged 18-49 and > or =50 years, respectively. The booster effect in subjects with only the primary vaccination course prior to study entry clearly depended on the time elapsed since last TBE vaccination with an estimated annual decline rate of 15%. In subjects with at least one additional booster vaccination virtually no antibody decline was observed. This study clearly indicates that (1) adults may be effectively and safely boosted with a different TBE vaccine and (2) following four immunisations protective antibodies can be detected far beyond a period of 3 years, thus, strongly supporting the reconsideration of currently recommended booster intervals.