Summary and Conclusions
From the results of the studies described here, it is obvious that immediate application of titration in rabbit kidney tissue culture is a valid measure of smallpox vaccine potency only for the virus strain and passage levels employed. Some other variant of the virus could conceivably have an adequate tissue culture titer but a poor “take” rate in human beings. However, after establishing its correlation to the “take” rate in both man and rabbits, titration can be applied to any strain.
In our study, four lots of smallpox vaccine of calf lymph origin and seven of chick embryo origin were tested for potency by the standard rabbit scarification method and by titration in rabbit kidney tissue cultures and in chick embryos (LD50 method). With the virus and seed passage level used, all three methods detected the one vaccine lot below standard potency. Good agreement of results obtained for all lots indicates that either titration method could advantageously replace rabbit scarification as a test of smallpox vaccine potency.
