Serological response to tick-borne encephalitis (TBE) vaccination in the elderly – results from an observational study

Abstract

Background: In the elderly, the incidence of tick-borne encephalitis (TBE) is higher than in younger individuals. At the same time, the responsiveness of the elderly to vaccination may be decreased. Product information for TBE vaccines therefore recommends determining antibody response after two of the three doses required for primary immunization in persons over 60. Objective: To determine the proportion of persons aged 60 years or older with insufficient antibody response after administration of two doses of TBE vaccine. Methods: A retrospective analysis of TBE-specific antibody levels as determined by Immunozym® ELISA in serum samples of 185 vaccinees > 60 years of age after the first two of the three vaccinations required for primary immunisation. Patients received either FSME-IMMUN Adult or Encepur® Adults at one vaccination centre in the Czech Republic. In a subset of 80 vaccinees, two additional ELISAs and a neutralization test (NT) were performed. Results: Overall, 33 of the 185 individuals (18%) included in the Immunozym analysis had antibody concentrations below putative protective levels after two doses. Geometric mean concentrations and seropositivity rates were higher in individuals vaccinated with FSME-IMMUN than in those vaccinated with Encepur®. While the differences in mean antibody levels and/or seropositivity rates in ELISAs for the Neudörfl strain (used in FSME-IMMUN) were significant, no significant differences were obtained for Enzygnost, which uses the Karlsruhe strain (used in Encepur). Conclusions: After two doses of TBE vaccine, one in five persons aged 60 years or older may be without protective antibodies, confirming the importance of testing the antibody response after the second vaccination and administering an additional dose in the event of insufficient response.