Sixteen trials were included in a Cochrane review involving over one million adults, children and infants. For killed whole cell vaccines compared to placebo, the relative risk of contracting cholera at 12 months was 0.52 (95% CI 0.42 to 0.65; n=1 512 573), i.e. an efficacy of 48%. Significant protection lasted for two years, even after only a single dose, and for three years with an annual booster. Children under 5 were only protected for up to a year.
Comment: The quality of evidence is downgraded by study quality (inadequate follow up).