Report of the 1966-67 cholera vaccine field trial in rural East Pakistan. I. Study design and results of the first year of observation.

A controlled cholera vaccine field trial was carried out in rural East Pakistan during the 1966-67 cholera season. A commercial cholera vaccine of average potency was tested in 40 000 children aged 3 months to 14 years in 1- and 2-dose schedules. In the cholera season extending for 8 months following immunization, a single dose produced an over-all protection of 46%; 2 doses at an interval of 1 month provided 64% protection. The single dose was virtually ineffective in children under 5 years, but provided significant protection in older children. The enhanced effect of the 2-dose schedule was primarily due to the boosting of protection in children under the age of 5 years. The duration of significant protection, even with the 2-dose schedule, did not appear to extend beyond the first 3 months of the 8-month cholera season.