Variation in Potency of Vaccinia Virus in Tissue Cultures

Abstract

The possibility of employing vaccinia virus in pure culture for prophylactic vaccination against small-pox has been indicated by Rivers, 1 Herzberg 2 and Stevenson and Butter. 3 This leads to investigation of the behavior of such cultures, especially their immunizing and infective properties. Therefore a study was undertaken to determine the effect of repeated passages in chick-embryonic tissue culture on the potency of the vaccinia virus.

The initial virus was prepared by passing the calf-lymph vaccinia virus supplied by the National Epidemic Prevention Bureau, Peiping, through the testicles of a normal rabbit. The bacteria-free vaccinia virus thus obtained was cultivated in the chick embryonic tissue according to the method described by Rivers. 1 Transfers of the virus to fresh chick embryonic tissue were made every 5 days. After each transfer the titre of the virus, i. e., the lowest dilution which still gave the typical skin lesion was determined by intradermal injections of 0.25 cc. of varying saline dilutions of the virus culture into the shaved skin of albino rabbits. The first virus culture gave a titre of 1:1,000 which rose to 1 :100,000 on the 3rd and 4th transfers and reached the peak of 1:1,000,000 on the 5th transfer after which it declined as subsequent transfers were made so that by the 25th transfer the titre had dropped to 1 :1,000, equaling that of the initial virus, and by 30th transfer it had fallen to 1:100. From this point on the titre remained at a level between 1:50 to 1 :100 when carried up to the 40th transfer. As no further drop in titre was observed, it was concluded that the virus culture had reached a stage of biological stability at the low titre of 1:50 to 1 :100.