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[NEW YORK] Oral cholera vaccines (OCV) provide substantial protection for adults but, for poorly understood reasons, they do not work so well for under-five children, says a review of clinical trials and observational studies.

The meta-analysis, carried out by the Johns Hopkins Bloomberg School of Public Health, Baltimore, looked at seven clinical trials and six observational studies to find that the standard two-dose oral vaccine regimen reduced the risk of cholera for adults by 58 per cent on average, but only by 30 per cent in under-five children. The findings were published July in The Lancet Infectious Diseases.

“We really don't know why the vaccine does not work as well in young children (under five) as it does in adults, though this is not a problem unique to OCV.”

Andrew Azman, Johns Hopkins Bloomberg School of Public Health

Cholera vaccines — which use killed, whole cells of the bacteria Vibrio cholera — became commercially available in the 1980s.

“These vaccines have not been widely available, have been fairly expensive for broad public health use and have been associated with a number of misconceptions, including that they don’t work well,” says Andrew Azman, epidemiologist at Johns Hopkins. “We decided to do this study to better summarise the evidence related to the protection from OCV.”

“We really don't know why the vaccine does not work as well in young children (under five) as it does in adults, though this is not a problem unique to OCV,” stresses Azman. “Similar issues have been noted with other oral vaccines like the rotavirus vaccine. More work is needed to understand how to provide better direct protection to children.”

Despite the lower protection of OCV on young children, Azman sees value in using them.

“We have seen in some studies that even though children may not directly benefit as much from the vaccine, they do benefit from 'herd-protection' when others in their family are vaccinated,” he emphasises. He also observed that one dose of these vaccines appears to provide similar protection as a two-dose regimen, at least within six months of vaccination.

“However, the standard two doses work well in the general population for at least three years. The choice of using a one-dose or two-dose regimen often confront public health officials during outbreaks in regions where supplies are scarce,” he says.

Daniel Leung, professor in the infectious diseases division, University of Utah, Salt Lake City, says the group’s work, which combined data from both older and more recent trials, generated updated estimates of OCV’s efficacy and effectiveness. “It is an important contribution to the evidence base for the use of OCV in both endemic and outbreak settings.”

Leung says the findings “strengthen the case for use of a single dose to provide short-term protection, and highlight the need for improved vaccines with higher efficacy in children under five years of age”.
This piece was produced by SciDev.Net’s Asia & Pacific desk.