Use of the single radial immunodiffusion test as a replacement for the NIH mouse potency test for rabies vaccine

The method currently recommended by the World Health Organization (WHO) for the potency assay of rabies vaccine is the NIH mouse potency test, a highly variable test requiring large numbers of animals. The Single Radial Immunodiffusion (SRID) test, an in vitro test, has been used successfully for the quantitation of hemagglutinin in inactivated influenza vaccine and is being evaluated for its utility as an assay for the rabies virus glycoprotein, considered to be the major protective antigen, of rabies vaccine. Potency values calculated using the SRID test were compared with those calculated using the NIH test for rabies vaccines produced in cell culture. The within-test variability was significantly lower with the SRID test but the potency values were generally higher than those from the NIH test. Vaccines which assay below the minimum acceptable potency value (2.5 International Units/ml) in the NIH test generally gave values above that level in the SRID test. The implications of these results on rabies vaccine control testing are discussed.