Comparison of Titrations on the Chorioallantoic Membrane of Chick Embryos with the Rabbit Scarification Technique for the Potency Assay of Smallpox Vaccines

The potency of the U.S. Reference Smallpox Vaccine, Lot 2, the International Reference Preparation of Smallpox Vaccine, and commercial smallpox vaccines was determined by the chorioallantoic membrane (CAM) and rabbit scarification (RS) potency assay methods. The mean titer of the U.S. Reference (based on 107 ampoules) was 108.1 pock-forming units (PFU) per ml and that of the International Reference (based on 3 ampoules) was 107.8 PFU/ml. A statistical analysis of the CAM data for the U.S. Reference resulted in the establishment of a table of limits of acceptance for smallpox vaccines. Of the commercial smallpox vaccines tested by the CAM and RS methods, 89% demonstrated potencies comparable to the U.S. Reference. Our results show that the CAM test method has application in the control testing of smallpox vaccines produced by U.S. licensed manufacturers provided it is used within the limits discussed.