Comparison of rabies humoral antibody titers in rabbits and humans by indirect radioimmunoassay, rapid-fluorescent-focus-inhibition technique, and indirect fluorescent-antibody assay.

Rabies humoral antibodies were induced in eight New Zealand rabbits by a single intramuscular injection of inactivated suckling mouse brain rabies vaccine. The primary response to immunization was measured in blood samples taken at selected intervals for 6 months. The anamnestic response was measured in blood samples obtained 2 weeks after the rabbits received a booster immunization. The humoral antibody concentrations were measured by the rapid-fluorescent-focus-inhibition technique (RFFIT), indirect fluorescent-antibody assay (IFA), and indirect radioimmunoassay (RIA). The maximal neutralizing antibody titers as measured by RFFIT were attained by the 4th week and persisted into the 24th week. After booster immunization the antibody response was almost 10-fold higher than the highest level attained in the primary response. The antibody levels as measured by IFA and RIA were similar, but the titers as measured by either procedure were almost 10-fold lower than those determined by RFFIT. After booster immunizations the antibody levels, as measured by IFA and RIA, were three- and sixfold higher, respectively, than the maximal levels attained in the primary response. Twenty-two human serum specimens were tested by the same serological procedures, with disparate results. Both RIA and RFFIT effectively differentiated antirabies-positive sera from antirabies-negative sera.