Laboratory and preliminary clinical characterization of Vi capsular polysaccharide-protein conjugate vaccines

To improve its immunogenicity for children and adults and to make it suitable for routine immunization of infants against typhoid fever, the capsular polysaccharide of Salmonella typhi (Vi) was bound to the B subunit of the heat-labile toxin (LT-B) of Escherichia coli or the recombinant exoprotein A (rEPA) of Pseudomonas aeruginosa. The conjugates elicited higher levels of antibodies (micrograms per milliliter of serum) in mice and in guinea pigs than did Vi and, unlike Vi alone, elicited booster antibody responses in both species. In adult volunteers, Vi-LT-B and Vi-rEPA, respectively, elicited higher levels of antibodies than Vi alone after the first injection (4.74 versus 1.77 and 4.91 versus 1.77; P < 0.005) and 26 weeks later (2.32 and 2.69 versus 0.54; P < 0.04); a second injection of the conjugates did not elicit a booster response of Vi antibodies. None of the 51 vaccinees had fever or significant local reactions. Vi-rEPA elicited slightly higher levels of Vi antibodies than did Vi-LT-B at all intervals after injection, but these differences were not significant. Each conjugate elicited antibodies to its carrier protein. The antibody responses elicited in adults by Vi bound to LT-B and rEPA are similar to those of other polysaccharide-protein conjugates. These conjugates promise to be an improved Vi vaccine. Studies of Vi conjugates with adults and infants in areas where typhoid is endemic are planned.