Live, orally administered, attenuated vaccine strains ofVibrio choleraehave many theoretical advantages over killed vaccines. A single oral inoculation could result in intestinal colonization and rapid immune responses, obviating the need for repetitive dosing. LiveV. choleraeorganisms can also respond to the intestinal environment and immunological exposure toin vivoexpressed bacterial products, which could result in improved immunological protection against wild-typeV. choleraeinfection. The concern remains that live oral cholera vaccines may be less effective among partially immune individials in cholera endemic areas as pre-existing antibodies can inhibit live organisms and decrease colonization of the gut. A number of live oral cholera vaccines have been developed to protect against cholera caused by the classical and El Tor serotypes ofV. choleraeO1, including CVD 103-HgR, Peru-15 andV. cholerae638. A number of live oral cholera vaccines have also been similarly developed to protect against cholera caused byV. choleraeO139, including CVD 112 and Bengal-15. Live, orally administered, attenuated cholera vaccines are in various stages of development and evaluation.