Immunogenicity and feasibility of intradermal vaccination against rabies in Quebec

Preexposure vaccination against rabies is recommended for some travellers and individuals exposed to the virus through their work. At a cost of at least $150 per intramuscular (IM) dose, few follow this recommendation. In Canada, provided certain conditions are met, the National Advisory Committee on Immunization (NACI) and the Comité d'immunisation du Québec allow a more economical alternative, intradermal vaccine administration (ID) which uses 1/10 the IM dose. The purpose of this study is to assess the feasibility and immunogenicity of intradermal preexposure vaccination.

Students and employees at the Faculty of Veterinary Medicine received three doses of ImovaxRage™ (Sanofi Pasteur) inactivated, human diploid cell rabies vaccine at days 0, 7 and 21 or 28. An IM or ID booster dose was administered after two years when indicated.

Among the 159 participants who received three doses, 139 underwent serological testing in the year following vaccination and all achieved protective antibody levels. The antibody level was higher when measured within five weeks of the third dose. When the serological control was performed two years later, 65% of participants had a <0.5 IU/ml titre. Of the 22/30 participants who chose an ID booster, 100% responded and the average antibody titres were multiplied by 11, indicating a strong anamnestic response.

ID rabies vaccination is immunogenic, economic and could be considered for the booster dose. Protective antibodies decline rapidly after primary immunization by ID, so it would seem prudent to perform a serological control one year later on individuals at high risk of occult occupational exposure. An alternative would be to give these individuals a routine ID booster dose one year after primary vaccination, which would simplify initial treatment and reduce related costs (follow-up, blood sampling, serological tests, etc.). The persistence of protective antibodies after this booster dose should be assessed to determine the need for subsequent serological tests and the ideal interval between tests.