Findings could help inform policy decisions on how to use vaccines in outbreaks.
A new review of the research literature led by researchers at the Johns Hopkins Bloomberg School of Public Health shows that cholera vaccines provide substantial protection for adults but provide significantly less protection for children under age 5.

The review, which considered seven clinical trials and six observational studies, found that the standard two–dose vaccine regimen reduced the risk of getting cholera on average by 58 percent for adults but only by 30 percent for children under age 5.

The findings appeared online in the journal The Lancet Infectious Diseases.

Vaccines for cholera cost an average of $1.85 per dose.

Oral vaccines for cholera, which are composed of killed, whole cells of the bacteria Vibrio cholerae, became commercially available shortly after their development in the 1980s. Yet for years, explains study leader Andrew Azman, PhD, research associate in the Department of Epidemiology at the Bloomberg School, 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.

Studies have suggested a wide range of how well these vaccines provide protection, with some results suggesting that the risk of cholera is cut in half with the vaccine and others suggesting that risk is nearly eliminated.

“There continues to be a lot of misinformation on what this vaccine is and what it can do,” Azman says.

To get a better grasp on the abilities of these vaccines and to help understand discrepancies in study results, Azman and his colleagues from the Oral Cholera Vaccine Working Group of The Global Task Force on Cholera Control did a literature review and meta–analysis on the vaccines’ efficacy (how well they protect in ideal situations, such as a randomized clinical trial) and direct effectiveness (how well they work in less ideal situations, such as in the midst of an outbreak).

The team searched for any randomized controlled trials and observational studies that examined this data for oral cholera vaccines. Their results turned up seven trials and six observational studies, each of which involved the three major commercial vaccines currently in use. The trials and studies involved more than 500,000 participants combined.

Taking an average of these results, the researchers found that for a two–dose regimen – the standard for these vaccines – efficacy was 58 percent and effectiveness was 76 percent. However, for children younger than 5, efficacy was substantially lower: around 30 percent. One dose of these vaccines appeared to provide similar protection as a two–dose regimen, at least within the six months following vaccination. There were no data to examine long–term protection of a one–dose regimen.

The review helps explain discrepancies in past studies that examined how well these vaccines work, say the authors. For example, efficacy for cholera vaccines tends to be lower than effectiveness, a scenario that’s opposite from that of most other vaccines and one that has puzzled researchers. However, Azman explains, because efficacy studies tend be conducted in places where cholera is more common in young children – a population in which these vaccines aren’t as effective – that may explain why efficacy is lower than effectiveness.

The paper also offers hope of using a one–dose regimen in areas experiencing high cholera transmission, such as the current outbreak in Yemen. A one–dose approach may have been a more effective way to control the outbreak than providing two doses to half the people.