Comparisons were made between three methods for assaying the potency of smallpox vaccines. After the elimination of refractory animals, the rabbit scarification test proved to be as reproducible as titration in bovine embryonic skin tissue culture. Both these tests were more reproducible than infectivity titration in the egg.

Equations relating infectious units determined by the three tests were determined. An over-all correlation of 94% was obtained for the acceptance-rejection records of 126 vaccine samples when titration in the rabbit and in bovine embryonic skin were compared. Seventy-seven of the vaccines were acceptable on the basis of each test. Correlation between the egg and rabbit tests was 81%.

Stability studies on a lyophilized vaccine utilizing the several titration methods demonstrated, with remarkable consistency among tests, that the vaccine was stable for at least 18 months under vacuum or nitrogen.

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