Laboratory and vaccination studies with dried smallpox vaccines.

In a vaccination and laboratory study, two dried smallpox vaccines (designated P and Q) were tested at intervals of 4, 8, 16, and 32 weeks after storage at both 37 degrees C and 45 degrees C. Vaccine P was also tested after 64 weeks at these temperatures and gave 100% successful vaccination rates after all periods of storage at both temperatures. Vaccine Q deteriorated within four weeks, rapidly at 45 degrees C and less rapidly, but very substantially, at 37 degrees C. There was no clear evidence of the cause of this deterioration, but there was a suggestion of denaturation of some of the samples stored at the higher temperature. So far as could be ascertained, the laboratory results-rabbit skin scarification tests and chorio-allantoic membrane pock counts-ran parallel with the vaccination success rates. The pock count was found to be the more accurate method of laboratory titration. Vaccine P as used in the trial was not an exceptional batch.Vaccines which give a pock count of 10(8) infective units per ml will give the highest possible rate of successful primary vaccinations.A statistical note on the trials is given in an annex.