Improvements in the rabbit scarification method for the assay of smallpox vaccine

For many years in the USA and at least eighteen other countries in the world, the rabbit scarification method, with modifications, has been the laboratory test system used in assaying potency and in standardizing smallpox vaccines. A revaluation of this control procedure has led to further attempts to improve the method and to the studies described in this paper. Refinements in the method include an improved scarifying instrument permitting more uniform vaccination results, a modified rabbit testing system using an increased number of animals, a calibrated gradient ensuring more precise measurement of individual pocks and confluent areas, and a simple mathematical scheme for quantitating potency of unknown vaccines in terms of a reference. Experimental data from 500 rabbits reflect the precision of the test system with the refinements suggested. A brief discussion of the advantages and disadvantages of the rabbit scarification test system as against other vaccinia virus potency titration methods indicates how the improved scarification method could better serve as a practical and satisfactory means for evaluating the relative potency of unknown vaccines in the control laboratory.