Purified Vi Antigens as Possible Typhoid Vaccines

Two purified samples and one crude preparation of Vi antigen were studied to determine their potential as typhoid vaccines. Each preparation was characterized by mouse toxicity studies, pyrogen tests in rabbits, and active protection tests in mice. Both purified samples of Vi antigen were nontoxic and nonpyrogenic in the highest doses tested (500 üg). In contrast the LD50 for the crude Vi antigen preparation was approximately 500üg, while the minimum pyrogenic dose was approximately 0.1 üq. In protection studies each Vi antigen preparation was compared with commercially obtained, heat-killed, phenol-preserved (HP) typhoid vaccine. With doses of 1 üg or 10 üg for the appropriate Vi antigen, mice were protected to an extent comparable to, or, in most instances, significantly greater than were mice receiving a standard dose (0.5 ml) of the HP vaccine. Since it elicits no toxic or febrile response in doses which evoke a high degree of protection in the standard bioassay in mice, purified Vi antigen represents a favorable alternative to the currently employed HP vaccine.