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A single dose of an oral cholera vaccine that's usually given twice might be enough to slow an epidemic, researchers reported.

In a randomized placebo-controlled trial, one dose of the vaccine Shanchol had a protective efficacy of 40% in a region of Bangladesh where cholera is endemic, according to, of the icddr,b (formerly known as the International Centre for Diarrheal Disease Research, Bangladesh) in Dhaka, and colleagues.

The vaccine's protective efficacy against severely dehydrating cholera was 63%, Qadri and colleagues .

On the other hand, the vaccine did not appear to protect children under the age of five, perhaps because they have not yet developed the additional resistance to cholera that is expected in a population where the disease is endemic, the researchers said.

But for older children and adults a single dose "seems sufficient at least for short-term protection," the researchers argued, so that in an epidemic the challenges of operating a two-dose vaccination program "need not be a consideration."

Cholera remains an important problem in many parts of the world and often emerges in the wake of natural disasters, as occurred in Haiti after the Jan. 12, 2010, earthquake. The outbreak, blamed on bacteria imported by rescue workers, sickened more than 600,000 people and killed nearly 7,500.

The Shanchol vaccine, licensed in 2009 by India's , is a killed whole-cell-only oral drug and costs $1.85 per dose, making it affordable for wide use in developing countries.

One clinical trial showed that two doses of the vaccine conferred 5-year cumulative protection of 65%, while another found 53% protection over 2 years. On the basis of those trials, the of the drug for use in cholera epidemics.

But, Qadri and colleagues noted, the drug produces "substantial immunologic responses" after the first dose and the question arose whether one dose might be enough to blunt epidemics arising after natural disasters.

To help answer that question, they conducted a randomized trial among residents of a slum district of Dhaka, where endemic cholera cases typically peak in March and April. Some 204,700 people got a single dose of vaccine or placebo from Jan. 10 to Feb. 4, 2014.

The primary outcome of the trial was culture-confirmed cholera that occurred between 7 and 180 days after dosing, and a key secondary endpoint was the proportion of people with severely dehydrating disease over the same period. The investigators also looked at protective efficacy by age.

Over the 6 months of follow-up, the researchers reported, there were 101 first cholera episodes, including 37 with severe dehydration, and the calendar timing and caseload were similar to what has been observed previously in the region.

All of the cases were caused by the Vibrio cholerae O1 El Tor biotype, they reported.

Of the 101 cases, 63 occurred in the placebo group and 38 among those who had been vaccinated, for respective rates of 0.62 cases per 1,000 placebo recipients and 0.37 cases per 1,000 vaccine recipients.

Those figures yielded a protective efficacy of 40% (95% CI 11%-60%), Qadri and colleagues reported.

Of the 37 severe cases, 27 were among placebo recipients and 10 among those who got the vaccine, for respective rates of 0.26 and 0.10 cases per 1,000 participants, and a protective efficacy of 63% (95% CI 24%-82%).

The efficacy was 16%, 63%, and 56% against all cholera episodes, respectively, among people one to four, five to 14, and 15 or older, but the differences between age groups were not statistically significant.

The vaccination program was safe, Qadri and colleagues reported, with adverse events similar in the two groups.

The investigators cautioned that the study was conducted in a region where cholera is endemic and residents have some natural immunity to cholera, so that it's not clear how well the findings will apply to populations without such immunity.

As well, they noted, the findings only apply to short-term protection.

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