A controlled field trial of live Salmonella typhi strain Ty 21a oral vaccine against typhoid: three-year results.

A controlled field trial of Salmonella typhi strain Ty 21a oral vaccine against typhoid was carried out in Alexandria, Egypt, from March 1978 to March 1981. A total of 32,388 children was included in the study. The children were divided in two comparable groups, one given three doses of vaccine and the other three doses of placebo. Each dose of vaccine contained 1-8 X 10(9) live Ty 21a bacteria. The population was monitored, and each suspected case of typhoid was investigated bacteriologically and serologically. The effectiveness of the vaccine was assessed by analyzing the number of confirmed cases of typhoid fever in the two groups. The incidence of typhoid fever was 4.9 cases per 10,000 children per year in the control group and 0.2 cases per 10,000 children per year in the vaccine group. The results indicate that, in the dose schedule used, the Ty 21a mutant strain, which is stable and safe, is protective for a period of at least three years.