Relationship between pock counts on chorio-allantoic membrane and percentages of "takes" in primary vaccination of human beings with two smallpox vaccines.

In view of doubts as to the potency level required to obtain nearly 100% successful vaccinations with the glycerolated smallpox vaccine prepared by the Netherlands National Institute of Public Health, its dose-effect relationship was compared with that of a glycerolated lymph from the Lister Institute of Preventive Medicine, England. The potency was estimated from pock counts on the chorio-allantoic membrane of chick embryos and the effect was judged from the "take" rate obtained in the primary vaccination of soldiers, the data being subjected to probit analysis.The relationship observed between the number of pock-forming units and the percentage of successful primary vaccinations was nearly identical with the two vaccines. The 50% effective doses were found to be 1.3-1.4 x 10(6) PFU/ml and the 99% doses to be 4.1-4.3 x 10(7) PFU/ml.