A controlled field trial of the effectiveness of various doses of cholera El Tor vaccine in the Philippines.

A controlled field trial of the effectiveness of various doses of cholera El Tor vaccine was organized in Negros Occidental Province, an area of endemic cholera in the Philippines, in 1966 and 1967, on 359 600 volunteers. The population was divided, by random allocation, into four equal groups, and vaccines were administered as follows: Group A, one dose of 1 ml of vaccine containing 8 x 10(9) organisms/ml; Group B, 2 doses (1 ml each) of the same cholera vaccine; Group C, one dose of vaccine containing 16 x 10(9) organisms/ml; and Group D, 2 doses of typhoid vaccine as a control.During the follow-up period of 6 months, a total of 95 bacteriologically proven cases were observed. The rates of protection were 53%, 55% and 58% respectively, which were statistically significant. Maximum protection was observed in Group C, but the difference between the degree of protection in the various groups was not statistically significant.