Early rabies antibody response to intramuscular booster in previously intradermally immunized travelers using human diploid cell rabies vaccine

Background: Postexposure treatment (PET) of travelers who may have had a po tential rabies exposure is simpler, safer, and cheaper if the traveler is p reimmunized. Preimmunization can be done with human diploid cell rabies vac cine (HDCV) administered intramuscularly or intradermally. Some authorities , however, are now advocating that travelers vaccinated by the intradermal (ID) route should be treated as if they are not immunized. A particular con cern raised is that travelers who have received pre-exposure rabies vaccina tion intradermally, may have a delayed response to postexposure boosters. T his study is designed to elucidate whether a single intramuscular (IM) HDCV booster will provoke an early (day 5) immune response in individuals given pre-exposure ID HDCV. Methods: Twenty-nine travelers who had received a course of three 0.1 mL ID HDCV between 12 and 24 months previously were given a single 1.0 mL IM boo ster of HDCV. Rabies antibody levels were compared 5 days later to those be fore the booster. Results: Twenty-five of the 29 subjects (86%) showed an adequate rise in vi rus neutralizing antibody (VNA) titer 5 days after booster. Nine of the 29 subjects (31%) had inadequate antibody levels prior to the simulated postex posure booster. Five days after the postexposure booster, 27 of 29 (93%) ha d adequate antibody levels. The other 2 travelers were subsequently shown t o have adequate VNA levels when tested 4 and 6 weeks later, respectively. Conclusion: For travelers who were given pre-exposure ID HDCV vaccination w ithin the last 2 years and received one IM postexposure booster dose of HDC V, most mounted an adequate early immune response. This data does not suppo rt a change in current recommendations for rabies PET in this group. Furthe r research to ascertain the duration of protection of pre-exposure ID rabie s immunization is required.