Study of live typhoid vaccine in chimpanzees

Streptomycin-dependent Salmonella typhi containing O and H antigens was administered as a live oral antityphoid vaccine to chimpanzees. Five animals served as controls; 5 others received the vaccine 4 times at 3-day intervals; 4 further animals were given 4 doses of vaccine at 3-day intervals together with streptomycin; and 1 animal received the 4 doses of vaccine and a daily dose of streptomycin. The individual vaccine doses varied between 36x10(9) and 82x10(9) organisms, totalling about 258x10(9)Salm. typhi per animal. The chimpanzees were challenged with 26x10(9) cells of the virulent Salm. typhi Ty2 strain 10 days after immunization and followed up bacteriologically, serologically and clinically. It was observed that after this very heavy challenge the immunized animals that had received streptomycin with the vaccine were protected to some degree against the infection and showed fewer symptoms. The animal that received vaccine and streptomycin daily did not develop the disease.The authors point out that, while the strain used may have potential usefulness for the protection of man, further studies are needed to confirm the innocuity of the vaccine, to reduce the strain's reversion to streptomycin-independence, and to determine the relative effectiveness of different immunization dosages and schedules.