The current pandemic of cholera is caused primarily by Vibrio cholerae O1 of the El Tor biotype. Live attenuated classical biotype V. cholerae vaccinestrains prevent severeand moderate cholera due to either biotype in challenged volunteers but may provide less protection against mild cholera due to El Tor organisms. CVD 110, a new ctxA-deleted vaccine strain derived from an El Tor Ogawa parent, lacks zona occludens toxin (Zot), accessorycholera enterotoxin (Ace), and hemolysin/enterotoxin. Ten healthy adult volunteers were given 108 cfu of CVD 110 with buffer;7 developed diarrhea (mean stool volume, 861 mL). Vaccine organisms wereshed in stool by all vaccinees and were recovered from duodenal fluid in three-quarters of vaccinees. After vaccination, the geometric mean peak reciprocal vibriocidal titer among vaccinees was 17,829. CVD 110 is a powerful immunogen but insufficiently attenuated despite the absence of known potential enterotoxins of V. cholerae. Another unrecognized toxin or colonization alone may be responsible for diarrhea after ingestion of this strai
