Experiments with Vi-negative strains of Salmonella typhi.

The immunological properties of a number of strains of Salmonella typhi were tested. One of these strains, although it did not contain the Vi antigen, was able to protect the mice against Vi-positive challenge when used as a vaccine for mice. The finding of a stable Vi-negative strain is important, as many strains, such as the classic O 901, revert to the Vi-positive state on a single mouse passage.Using the Vi-negative organism as challenge in eight assays of the vaccines used in the Yugoslav field studies at Osijek, the phenolized vaccine was shown to be about 10 times more potent than the alcoholized vaccine. Further laboratory tests in conjunction with field trials will be necessary to show if these results are significant.