Abstract
The genes encoding the A (toxic) subunit of cholera toxin were deleted from pathogenic Vibrio cholerae 01 strain 569B by recombinant techniques, leaving intact production of immunogenic, non-toxic B subunit. The resultant strain, CVD 103, evaluated for safety, immunogenicity, and efficacy as a live oral vaccine, was highly attenuated and elicited strong antibacterial and antitoxic immune responses; a single dose significantly protected volunteers against challenge with pathogenic V cholerae 01 of either serotype or biotype. A further derivative, CVD 103-HgR, which has an Hg++_ resistance gene to differentiate it from wild-type vibrios, was also well-tolerated, immunogenic, and protective; moreover, faecal excretion of this derivative was significantly lower than that of CVD 103, which should minimise environmental spread of the vaccine. CVD 103-HgR is a candidate for expanded clinical trials in endemic areas.
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