Field trials of monovalent Ogawa and Inaba cholera vaccines in rural Bangladesh--three years of observation.

A controlled cholera vaccine field trial was carried out to test the efficacy of monovalent whole-cell Inaba and Ogawa cholera vaccines and a purified Inaba antigen. This study was designed particularly to study the level of protection produced by these vaccines against homologous and heterologous serotypes and to correlate the results with mouse protection tests and human serological response to the vaccines. A cohort of 45 000 children, aged 0-14 years, was divided into a control group and three vaccine groups. Inoculations were given annually for 2 years just before the start of the cholera season, and follow-up was continued for one additional year. Essentially, all cholera cases were due to the Inaba serotype, so that protection could be studied only against that serotype. Two annual injections of the whole-cell Inaba vaccine gave the highest level of protection, averaging 84% over the 3 years of follow-up; a single injection of the purified Inaba vaccine gave less protection (51%). Two annual injections of the whole-cell Ogawa vaccine failed to protect children under the age of 5 but did produce 48% protection for children aged 5-14 against Inaba cholera. Serological surveys correlated poorly with protection; specifically, the Ogawa vaccine produced high anti-Inaba titres in young children but no protection. The cross-protection against Inaba cholera produced by Ogawa vaccine in the older children is assumed to be due to boosting of naturally acquired immunity in this population. Monovalent vaccine cannot be recommended for general public health use because of the serotype specificity of protection that this study has demonstrated.