Vaccinating large numbers of people against cholera at the first signs of an outbreak could save hundreds or even thousands of lives, a new analysis of past epidemics in Zimbabwe, Zanzibar and India shows. Another study indicates that such immediate vaccination in Vietnam may have limited an outbreak there. Both studies appear in the January PLoS Neglected Tropical Diseases.
Although easily administered oral vaccines exist, public health officials typically don’t vaccinate against cholera in the throes of an outbreak because medical workers have their hands full rehydrating patients who have come down with the diarrheal disease. Besides, cholera historically moved on to new areas in a matter of months, well before a vaccine campaign could have an effect. But the Vibrio cholerae bacterium that causes the disease has morphed in recent years, now causing infections that can linger and extend a disease outbreak.
“Historically, the cholera vaccine has been secondary,” says immunologist Edward Ryan of Harvard University and Massachusetts General Hospital in Boston, a cholera expert who wasn’t involved in the new studies. Public health officials have concentrated on detecting cases, rehydrating patients, providing clean water and improving sanitation to stem the spread of cholera, he says.
“The changing features of the pandemic — and data like we’re seeing from these two studies — would suggest it may be time to revisit what role cholera vaccine could play in an outbreak,” Ryan says.
In one study, scientists collected information from three regions where cholera has struck in the past 15 years — Zimbabwe, Zanzibar and India.
In Zimbabwe, cholera killed more than 4,000 people and infected nearly 100,000 in 2008 and 2009. A computer-assisted analysis of the epidemic shows that a prompt campaign to vaccinate half the population would have prevented 40 percent of the cases and nearly 1,700 deaths, report epidemiologist Rita Reyburn of the International Vaccine Institute in Seoul, South Korea, and an international team of colleagues.
A analysis of seven outbreaks that struck the Indian Ocean islands of Zanzibar and Pemba (both part of Tanzania) between 1997 and 2004 shows that island-wide vaccination of half the population would have reduced cases by 4 to 29 percent, depending on the outbreak.
When applied to three outbreaks that hit Kolkata (Calcutta) from 2003 to 2005, the computer analysis showed that vaccinating 50 percent of the population would have prevented 36 percent of the cases.
The analysis assumed the availability of a cholera vaccine stockpile, enabling immunization of large numbers of people within about 10 weeks. Slower responses yielded estimates of fewer lives saved and infections prevented.
While a global vaccine cache currently exists for yellow fever, there is no similar stockpile for cholera. But Vietnam, where cholera has become common in the past decade, has its own stockpile. Public health officials there put it to use at the start of a cholera outbreak in Hanoi in 2007 and 2008, a move that appears to have prevented anywhere from 5 to 94 percent of possible cases, according to a separate report in the same PLoS Neglected Tropical Diseases issue. Dang Duc Anh of the National Institute of Hygiene and Epidemiology in Hanoi and colleagues identified 54 people who had cholera during the epidemic and 54 others who didn’t. People who did not get cholera were twice as likely to have been vaccinated compared with those who got sick.
Cholera spreads through contamination of drinking water by the fecal matter of infected people. The two studies didn’t account for additional reductions in the spread of disease that would come through vaccinating the population, which reduces fecal contamination of water supplies. Nor did they account for “herd immunity,” the protection that some people get because others around them are vaccinated and not infectious, Ryan says. “For these reasons, the effect we’re seeing might be low-balling what the benefit of vaccination would be in reality,” he says.
Reyburn cites the ongoing cholera epidemic in Haiti, in which people continue to be infected months after its onset. “The current response strategy, despite huge efforts, struggles to control outbreaks,” she says. “Mass oral cholera vaccination is a powerful new tool to complement clean water and sanitation and good case management. It should be utilized.”