Cholera Vaccines Found to Provide Suboptimal Coverage in At-Risk Population

As cholera continues to spread in Yemen, a review article from Johns Hopkins Bloomberg School of Public Health offers new insight into the cholera vaccine that could potentially be used to inform policy makers’ decisions on the best way to use the inoculation during outbreaks.

The review found that although cholera vaccines offer “substantial protection” for adults, they are not providing enough protection for children under 5 years—a population that, once infected, are at risk of dying from the diarrheal disease due to organ failure caused by dehydration.

It is estimated that worldwide, a staggering 3 to 5 million individuals are infected with cholera annually, and 28,000 to 130,000 individuals per year lose their lives to the disease. Because the disease is transmitted through a fecal—oral route, cholera outbreaks tend to occur in areas without access to safe, clean drinking water, and those with poor sanitation, and weakened healthcare systems. One such outbreak in ongoing in Yemen, where 400,000 suspected cases and at least 1,900 related deaths have occurred since April 2017, when the outbreak began.

Oral cholera vaccines have been commercially available since the 1980s; however, there are several misconceptions about the effectiveness of these vaccines. Some studies have boasted that an individual can cut their risk for cholera in half if they are vaccinated, while other research goes so far as to suggest that “risk is nearly eliminated” with the vaccine.

“There continues to be a lot of misinformation on what this vaccine is and what it can do,” study leader Andrew Azman, PhD, research associate in the Department of Epidemiology at the Bloomberg School stressed in a recent press release.

Therefore, Dr. Azman and his team set out to get a clearer picture of the vaccines’ efficacy, which they define as “how well they protect in ideal situations, such as randomized clinical trial,” and their direct effectiveness, or, “how well they work in less ideal situations, such as in the midst of an outbreak.”

The researchers turned to scientific literature comprising randomized controlled trials (RCTs) and observational studies that had been conducted in the past. They wheedled their review down to look at 7 RCTs and 6 observational studies; all of which included data on the 3 cholera vaccines that are currently available.

After averaging the RCT and observational study results, the researchers found that the standard 2-dose regimen for the vaccines boasted an efficacy rate of 58% and an effectiveness rate of 76%. However, vaccine efficacy was much lower in a key population at particular risk for the disease: children.

In fact, the efficacy rate for children under 5 years of age was a meager 30%.

Furthermore, the researchers stumbled upon another interesting discovery: protection provided by a single dose of the vaccine in this population was comparable to the recommended 2-dose regimen. Meaning that, regardless of whether the children received the single dose or the 2-dose regimen, there was no substantial difference regarding efficacy of the vaccine.

The authors posit that their review clears up a few discrepancies gleaned from past studies on the efficacy and effectiveness of the cholera vaccines. “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,” according to the press release. Dr. Azman theorizes that this may be because studies examining efficacy are oftentimes carried out in regions where cholera is a common disease that children are faced with and exposed to, “a population in which these vaccines aren’t as effective.” Therefore, Dr. Azman says that this could explain why efficacy is lower than effectiveness.

The results of the review also show that the available vaccines are not providing optimal protection for a key population at risk for the disease, findings that Dr. Azman feels “should have enormous implications for vaccine policy.” Due to these findings, researchers need to identify the best way to use the vaccines available in order to provide stronger coverage of young childrenm especially until a new vaccine regimen is developed. Meanwhile, adults who live in households with young children should ensure that they receive their recommended vaccinations.

Furthermore, the finding that a 1-dose regimen may be able to provide the same amount of protection as a 2-dose regimen can be monumental for regions continually hit by disease outbreaks. Cholera vaccines cost, on average, $1.85 per dose; but if the dosage is cut in half, then so are the associated costs. Dr. Azman suggests that by being able to give 1 dose instead of 2 in outbreak situations, health officials can potentially stretch out the supply to cover “twice the number of people.”

“In cholera-prone areas, public health decision makers don’t always have the luxury of caring about the outbreak risk in 2 years,” Dr. Azman explained. “They care about the risk of transmission tomorrow or in 2 weeks or next month. Being able to vaccinate twice the number of people with a limited quantity of vaccine can be very important.”