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Live oral cholera vaccine CVD 103-HgR is well-tolerated and immunogenic when administered to adults, school age children and preschool children in a single 5 × 109 colony-forming unit dose. Because elicitation of immune responses after administration of a single dose is exceptional for any oral vaccine in any age group, CVD 103-HgR was used as a probe to investigate the clinical acceptability, practicality and immunogenicity of this vaccine in infants and toddlers 3 to 17 months of age.The study was undertaken successively in 12- to 17-month-olds (n = 104), 7- to 11-month-olds (n = 106) and 3- to 5-month-olds (n = 102). One-half of the subjects were randomly allocated to receive vaccine and the other one-half to receive placebo, in double blind fashion. After 2 weeks of double blind follow-up, all subjects received a dose of vaccine. Vibriocidal antibody titers were measured on coded sera collected at baseline and 2 weeks after each dosing. The buffered vaccine "cocktail" had a volume of 100 ml; subjects who ingested ≥70 ml were considered fully vaccinated.Only 37% of subjects overall (25% of 3- to 5-month-olds) ingested ≥70 ml of the cocktail. The vaccine was well-tolerated with no significant differences in the rate or severity of adverse reactions after ingestion of vaccine vs. placebo. Seroconversion after ingestion of a single dose of CVD 103-HgR was similar in fully vaccinated subjects (66%) and in those who ingested a smaller fraction of the vaccine cocktail (63%). Of subjects who ingested two doses, 5 of 118 excreted vaccine organisms on Day 7 after the first dose vs. 0 of 118 after the second dose.Single dose oral CVD 103-HgR is well-tolerated and immunogenic in infants even when a partial dose is ingested. The buffered vaccine cocktail that is readily imbibed by. older children is not appealing to young infants, and improved vaccine formulations and delivery vehicles for immunizing infants must be sought.