Zanzibar Shows Cholera Vaccine Can Protect Even The Unvaccinated

By Richard Knox

Published September 4, 2012 at 1:23 PM EDT

A vaccine against cholera bacteria like these protected people in Zanzibar.

Cholera vaccine gives indirect protection to unvaccinated people in communities where a substantial fraction of the population gets the vaccine, a study in Africa shows.

The effect is called "herd immunity." It works because there are fewer bacteria circulating in communities where vaccination levels are relatively high.

The result comes from , an island state of Tanzania, where half the people in six rural and urban areas received two doses of oral cholera vaccine. For those who got it, the vaccine was 79 percent protective against the disease. But their neighbors who didn't get vaccinated had almost as much protection.

"This finding is good news for policymakers who have to deal with cholera in settings where...safe water supply and adequate sanitation cannot be established in a reasonable time frame," the study authors write in The Lancet Infectious Diseases.

They say that the direct and indirect effects of cholera vaccination could eventually eliminate the disease in communities with regular vaccination programs. This is in line with a 2007 prediction by statisticians, but it now has the authority of real-world results.

The results give a boost to those who support much wider use of cholera vaccine in places like Haiti where cholera is endemic, meaning that periodic outbreaks can be expected.

Because the vaccine confers herd immunity as well as direct protection, the findings suggest that all 9.5 million Haitians would not need to be vaccinated in order to achieve a big reduction in cholera caseloads.

"Far from an absolute need for complete coverage," the authors of the Zanzibar study say, "reasonable coverage might well be sufficient to interrupt the transmission of cholera."

They add that is is "becoming increasingly unconscionable" not to include oral cholera vaccine in the public health response to the disease.

Last month the World Health Organization said it will build a global stockpile of cholera vaccine to help blunt outbreaks, with a longer-term goal of universal vaccination in endemic areas as manufacturers ramp up production.

The WHO decision was influenced by a pilot project in Haiti this past spring that showed it was possible to achieve more than 90 percent vaccination rates in two target populations, one rural and one in a Port-au-Prince slum.