Abstract
The role of Vi antigen in human immunity against typhoid fever has been debated for decades. Circumstantial evidence indicates that Vi antigen may play a role in pathogenicity and immunity. A Vi preparation was isolated from Salmonella typhosa, the causative organism in human typhoid fever, by a mild precipitation method. It was significantly more potent in animal studies than preparations from Citrobacter used in the past for human study and less toxic than conventional typhoid vaccines. With this antigen, the role of Vi antigen in human protection is now being investigated.