A Proposed Potency Test for Smallpox Vaccine. I. Evaluation of Chick Embryo LD50 Method

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Abstract
A glycerinated control smallpox vaccine of calf lymph origin was titrated 28 times, over a period of 2 years, by the chick embryo LD50 method. Calculations of LD50 titers were made at the end of 5, 6, 7 and 8 days'' incubation; those on the 7th day were optimal. Individual titers on the 7th day ranged from 10-6.7 to 10-8.4, giving a geometric average titer of 10-7.5 for the control vaccine with a standard deviati n of 0.33 log units. No significant seasonal variations of the average LD50 titers were noted. The precision of the method and its applicability to the potency testing of smallpox vaccine are discussed.