A new typhoid vaccine composed of the Vi capsular polysaccharide.

Typhoid is still prevalent in many parts of the world. We reviewed all published and unpublished studies of a newly licensed vaccine composed of the Vi capsular polysaccharide of Salmonella typhi, the causative agent of the disease, which had been licensed previously outside the United States. These included observational studies and double-blind randomized studies done in the United States, Europe, and the developing world in which children and adults unexposed to typhoid or those living in endemic areas were enrolled. A single dose of 25 micrograms of the purified polysaccharide was given by intramuscular injection. The vaccine was well tolerated, inducing only minor reactions in fewer than 10% of subjects. An antibody response occurred in about 90% of subjects and lasted about 3 years. Seroconversion was shown in children as young as 2 years. Protective efficacy was evaluated in two studies conducted in areas in which typhoid is endemic; the efficacy was 55% and 75%, respectively, in adults and in children older than 5 years. The Vi vaccine compares favorably with other typhoid vaccines in regard to safety, patient compliance, immunogenicity, and efficacy. Vi polysaccharide is a well-standardized antigen that is effective in a single parenteral dose, is safer than whole-cell vaccine, and may be used in children 2 years of age or older.