Potency testing of diploid-cell rabies vaccine and correlation with immunogenicity

Two procedures for evaluating the potency of diploid-cell rabies vaccine (WRV) were compared by mouse-serum neutralization (MNT) and RFFIT serological determinations, using NIH Reference Rabies Vaccine No. 182 as base. Mean antibody-induction potency value ratios in female CD1 mice for 12 WRV assays were 5.12 by MSN and 4.57 by RFFIT compared with a 2.84 Antigen Value by the standard 28-day NIH mouse neutralization tests on the same lots. The mean antibody-induction potency values for 12 control DEV preparations were 0.71 (MSN) and 0.81 (RFFIT) versus 0.51 Antigen Value by NIH test. The statistical parameters of all respective means were similarly compared. MRV potencies of the order of magnitude shown correlated well with superior human immunogenicity but not linearly over a ten-fold range of antigen values from about 1 to 10. MNT antibody responses were 15-30 I.U. per ml of serum to WRV in this range (3-dose immunization); corresponding overall RFFIT values were 8 to 15 I.U. per ml. Control immunizations with DEV preparations yielded immune responses approximately one order of magnitude lower under comparable conditions.