Immune responses to an oral typhoid vaccine strain that is modified to constitutively express Vi capsular polysaccharide.

Protection against typhoid fever might be best achieved by a vaccine that stimulates IgG antibody to Vi capsular polysaccharide (Vi) in serum, IgG antibody to O antigen in serum, and cell-mediated immune responses. Live typhoid vaccines have not elicited anti-Vi antibody, presumably because Vi expression is highly regulated. CVD 909 is an oral attenuated typhoid vaccine candidate that is engineered to constitutively express Vi. In the present study, CVD 909, at doses of 10(6-9) cfu, was orally administered to 32 healthy adults, and immune responses were measured. Although many of the volunteers generated antibody-secreting cell responses to Vi, only 2 of the 32 volunteers generated anti-Vi IgG antibody in serum.