Oral immunization with Salmonella typhi Ty21a-based clones expressing Vibrio cholerae O-antigen: serum bactericidal antibody responses in man in relation to pre-immunization antibody levels.

Previous studies have shown that oral immunization with Salmonella typhi Ty21a-based clones expressing Vibrio cholerae O-antigen elicits serum antibody responses against the foreign polysaccharide in human volunteers. These responses are conveniently assayed by complement-dependent bacteriolysis of V. cholerae. In this report the bactericidal responses generated by two such clones are analysed in relation to the pre-immunization titres of various serum antibodies. A significant association was found, in that recipients with higher prevaccine titres of anti-V. cholerae bactericidal antibodies were less likely to register significant bactericidal responses following immunization. These results are discussed in relation to the concept of vaccine exclusion.