Report of the 1966-67 cholera vaccine trial in rural East Pakistan.

A controlled cholera vaccine field trial was carried out in rural East Pakistan to determine the efficacy of a cholera vaccine of average antigenic potency when used in a continuing programme with annual reimmunizations. A cohort of 40 000 children aged 0-14 years was equally divided into a control group and 3 vaccine groups. Inoculations of vaccine were given annually for 3 years just before the start of the cholera season, and follow-up continued for 2 additional years. The results indicate that there was increasing protection with reimmunization, reaching a maximum with 3 doses. One dose produced 43% protection, 2 doses 64%, 3 doses 81%, and 4 doses 76%. Protection was more sustained after reimmunization; being 50% and 39%, 1 and 2 years after the fourth injection, respectively. Serological surveys suggested a general parallel in the antibody response to vaccine and the level of protection achieved; however, the levels of vibriocidal antibody titres could not be related directly to levels of protection. The overall protection achieved with the 3-year programme of annual reimmunizations was 55% for the group receiving one inoculation annually, and 65% for the group receiving 2 inoculations in the first year followed by annual reimmunizations. When the costs and effectiveness of annual vaccine programmes are compared with those for cholera treatment centres, it becomes clear that the cholera vaccines now available are not appropriate alternatives to treatment in routine cholera control programmes.