Immune suppression induced by Vi capsular polysaccharide is overcome by Vi-DT conjugate vaccine

► Pre-exposure to Vi capsular polysaccharide suppresses immune response to Vi-DT conjugate. ► Two doses of conjugate overcome suppression induced by Vi polysaccharide. ► Vi stimulates existing memory cells but does not induce new memory cells. ► Vi-DT conjugate appears to behave as a slow release vaccine for the DT component. ► un-conjugated Vi in the presence of Vi-DT conjugate appears to have adjuvant activity.