Antibody persistence following booster vaccination against tick-borne encephalitis: 3-year post-booster follow-up.

In order to evaluate the long-term immunity 2 and 3 years after booster vaccination against tick-borne encephalitis (TBE) following time intervals 3 years and longer since last TBE immunization, 195 (mean age 50.5+/-15.1 years) and 240 subjects (mean age 47.0+/-15.4 years), respectively, who had received a single booster dose in a preceding study, returned for a serological follow-up. Antibody concentrations were measured by neutralization test (NT) and ELISA. Protective TBE antibody levels (NT>or=10) were noted in 96% (187/195) of the subjects at year 2 and 97% (232/240) at year 3 post-booster. At both years, GMTs (NT) of all age groups were above detection limit (>or=2), showing GMT-NT of 54.5 (95% CI: 47.7-62.4) and 69.1 (95% CI: 60.7-78.6), respectively. GMTs were significantly lower in vaccinees>or=50 years of age (p<0.001 for both years). Comparison of GMTs at years 2 and 3 post-booster vaccination with pre-booster values revealed similar antibody titers at all three reading points. Besides an expected rapid decline of TBE antibodies shortly after booster administration, the kinetic curve also suggested maintenance of high values of neutralizing TBE antibodies for a period longer than expected, thus confirming the recent recommendations regarding the extension of TBE booster intervals.