Antibody response to preexposure human diploid-cell rabies vaccine given concurrently with chloroquine

We conducted a randomized controlled trial to evaluate the antibody response of freshman veterinary students to intradermal human diploid-cell rabies vaccine administered concurrently with chloroquine, a drug frequently used for chemoprophylaxis against malaria. Fifty-one students who had not been vaccinated against rabies were enrolled: 26 received 300 mg of chloroquine base per week (the recommended dose for malaria prophylaxis); 25 did not receive chloroquine and served as controls. All subjects received 0.1 ml of rabies vaccine intradermally on days 0, 7, and 28. Chloroquine was administered weekly to the treatment group, beginning nine days before the first dose of vaccine and continuing until day 48. The mean rabies-neutralizing antibody titer for the chloroquine group was significantly lower than that for the control group on each day of testing--i.e., day 28 (P = 0.0094), day 49 (P = 0.0008), and day 105 (P = 0.0002)--although both groups had neutralizing antibody titers on days 49 and 105, according to the criteria of the Centers for Disease Control. The blood concentrations of chloroquine and desethylchloroquine (the major metabolite of chloroquine, which also has antimalarial properties) were negatively associated with log antibody titers. These results indicate that chloroquine taken in the dose recommended for malaria prophylaxis can reduce the antibody response to primary immunization with intradermal human diploid-cell rabies vaccine.