A contribution to the laboratory assay of typhoid vaccines.

In the laboratory assay of typhoid vaccines many questions still remain open. The results of experiments are at times ambiguous, and there is no satisfactory agreement between them and the results of the various field trials. The authors of the present paper have attempted to clear up some of the causes of discrepancy. They have concluded that in evaluating the efficacy of various types of vaccine much depends on the vaccine and challenge doses used. By employing low or high doses of vaccine, it is possible to obtain different results in respect of the qualities of the vaccines compared. The time interval between vaccination and challenge also has a certain influence.Experimentally infected mice were examined for the presence of Salmonella typhosa in the peritoneum and in the blood. The authors' experiments show that bacteraemia, observable 30 minutes after intraperitoneal challenge, represents an important indicator of vaccine efficacy. This approach enabled them to obtain clearer and more detailed information regarding the protective power of the vaccine employed than would have been revealed by the more death or survival of the test animals.