Long-term serological response and boostability of intradermal rabies immunization: A retrospective chart review.

Objectives: Rabies vaccines in Canada are currently labeled for intramuscular use only; however, intradermal vaccine administration may be equally effective. This study aims to assess the immediate and long-term (≥ 2 years) serological response and boostability of intradermal administration of rabies vaccine for pre-exposure prophylaxis.

Methods: This retrospective cohort study was conducted using data from electronic medical records at the Winnipeg Regional Health Authority Travel Health & Tropical Medicine Services Clinic. Individuals ≥ 18 years of age at time of vaccination who received at least 3 × 0.1 mL doses of intradermal rabies vaccine were included. All charts were reviewed to extract post-primary immunization and post-booster serology results, and t-tests were conducted to evaluate seroprotection (defined as serology ≥ 0.5 IU/mL).

Results: A total of 324 individuals received an intradermal rabies primary pre-exposure vaccination series with follow-up serology within 90 days. Seroprotection was observed in 96% (GMT = 3.36 IU/mL, 95% CI 3.07-3.68). Of these, 68 had serology results 1.8-2.5 years later, with seroprotection declining to 44% (GMT = 0.52 IU/mL, 95% CI 0.41-0.65). However, 100% seroprotection was observed in those who received an intradermal booster dose at ≥ 2 years with follow-up serology (n = 20, GMT = 4.7 IU/mL, 95% CI 3.7-6.2).

Conclusion: Intradermal administration of a primary pre-exposure rabies vaccine series demonstrates a strong serological response. While titers decline over time, the robust serological response demonstrated after a single intradermal booster vaccine dose suggests that a strong anamnestic immune response was maintained. This finding supports existing evidence that intradermal administration of rabies vaccines can be used effectively for rabies pre-exposure and post-exposure prophylaxis.