Retrospective, matched case-control analysis of tickborne encephalitis vaccine effectiveness by booster interval, Switzerland 2006-2020

Abstract

Objective: To estimate effectiveness of tickborne encephalitis (TBE) vaccination by time interval (<5, 5-10 and 10+years) postvaccination.

Design: A retrospective, matched case-control study PARTICIPANTS: Cases-all adult (age 18-79) TBE cases in Switzerland reported via the national mandatory disease reporting surveillance system from 2006 to 2020 (final n=1868). Controls-community controls from a database of randomly selected adults (age 18-79) participating in a 2018 cross-sectional study of TBE vaccination in Switzerland (final n=4625).

Primary outcome measures: For cases and controls, the number of TBE vaccine doses received and the time since last vaccination were determined. Individuals were classified as being 'unvaccinated' (0 doses), 'incomplete' (1-2 doses) or 'complete' (3+ doses). Individuals with 'complete' vaccination were further classified by time since the last dose was received (<5 years, 5-10 years or 10+ years). A conditional logistic regression model was used to calculate vaccine effectiveness (VE: 100 × [1-OR]) for each vaccination status category.

Results: VE for incomplete vaccination was 76.8% (95% CI 69.0% to 82.6%). For complete vaccination, overall VE was 95.0% (95% CI 93.5% to 96.1%). When the most recent dose was received <5 years prior VE was 91.6% (95% CI 88.4% to 94.0%), 95.2% (95% CI 92.4% to 97.0%) when the most recent dose was received 5-10 years prior, and 98.5% (95% CI 96.8% to 99.2%) when the most recent dose was received 10+ years prior.

Conclusions: That VE does not decrease among completely vaccinated individuals over 10+ years since last vaccination supports the longevity of the protective response following complete TBE vaccination. Our findings support the effectiveness of 10-year TBE booster intervals currently used in Switzerland.

Keywords: Breakthrough Infection; Duration; Prevention; Protection; TBE.

Figure 1

Study flow diagram. FOPH, Federal Office of Public Health; TBE, tickborne encephalitis.

Figure 2

Vaccine effectiveness by age. For each age group (18–39, 40–59 and 60–79), individuals were categorised as unvaccinated, incompletely vaccinated (1–2 doses), completely vaccinated (3+ doses) <5 years prior, 5–10 years prior or 10+ years prior and VE was calculated using the formula (VE=100×[1−OR]), with unvaccinated as the reference.

Figure 3

Time since last vaccination among vaccinated cases and controls. (A) Among incompletely vaccinated cases (n=75) or controls (n=279), the percentage of individuals (with 95% CIs) that received their last vaccine dose by indicated times. (B) Among completely vaccinated cases (n=76) or controls (n=1377), the percentage of individuals (with 95% CIs) that received their last vaccine dose by indicated times.