After a century-long absence, cholera might become endemic in Haiti. Credit: M. Tama/Getty Images

Vaccination would help to address the current cholera epidemic in Haiti, say US researchers. They believe the measure should be deployed alongside efforts to provide clean drinking water and improved sewage treatment.

At present, no such vaccination is planned. Experts disagree over whether vaccinating against cholera during an outbreak is effective. There is also a worldwide shortage of vaccine.

But using a mathematical model, epidemiologist Dennis Chao of the Fred Hutchinson Cancer Center in Seattle, Washington, and his colleagues found that, combined with basic public-health measures, vaccinating the 5% of Haitians most at risk of the disease would reduce the number of cholera cases by 11%. Vaccinating 30% of the population could cut cases by 55%, preventing 3,320 deaths.

"What we are saying is embarrassingly simple," says Chao. "If there is a limited amount of vaccine, obviously we should vaccinate people in areas with big numbers of the disease." The study is published in the Proceedings of the National Academy of Sciences1.

"I don't think there is any excuse not to vaccinate those at risk," adds Chao's co-author Ira Longini, an epidemiologist at the University of Washington in Seattle. But even vaccinating 5% of Haitians would require more doses - a million - than are currently available.

"Because of this epidemic and others around the world, we recommend that a comprehensive global plan for stockpiling and using an oral cholera vaccine be developed," says Longini.

Second wave?

Haiti's first cholera epidemic in a century began last October. As of 4 April, 274,418 Haitians had fallen ill and 4,787 had died, according to the Pan American Health Organization (PAHO), the regional arm of the World Health Organization (WHO) based in Washington DC.

Nationwide, the rate of new cholera cases has fallen, but some areas are seeing increases. This could indicate a second wave of disease, says Chao.

Last November, the United Nations doubled its estimate of the eventual size of the epidemic from 200,000 cases to 400,000.

There is no consensus among public-health experts as to whether the epidemic has plateaued and is dying down, or whether there will be a resurgence during the rainy season, which is beginning. In March, epidemiologists Jason Andrews of Harvard Medical School in Boston, Massachusetts, and Sanjay Basu of the University of California in San Francisco predicted that Haiti would see 779,000 cholera cases and 11,000 deaths by the end of November 20112.

The figure is plausible, says David Sack, a vaccine expert at Johns Hopkins University in Baltimore, Maryland, noting that previous epidemics have been far larger than initially predicted. "Cholera keeps surprising us," he says.

Vaccine shortage

One-quarter of Haitians live next to rivers, where the Vibrio cholerae bacterium seems to thrive. The long absence of epidemics means there is probably little or no natural immunity, and the strain causing the current epidemic is more virulent than those seen elsewhere in the past.

Cholera may well become endemic, says Chao. And vaccines confer immunity for only about 2.5 years, he adds, making revaccination necessary.

But even if it is agreed that vaccination would be effective, there are not enough doses in the world to treat all 10 million Haitians. Last December, Haiti's Ministry of Health decided not to vaccinate, a decision that it is standing by.

"At the beginning of the epidemic, vaccination was considered, but given the limited global vaccine supply and other issues [such as fairness], cholera vaccinating was not considered practical," says Carolina Danovaro, a regional immunization adviser at PAHO.

There are probably between 400,000 and 600,000 doses of oral cholera vaccine worldwide, says Longini. A course of vaccination requires two doses.

Of the two available vaccines, one, Dukoral, is expensive and difficult to produce but is approved by the WHO. The second, Shanchol, is much cheaper. It is licensed in India but still undergoing the WHO's approval process (see Cholera tightens grip on Haiti).

The Haitian government could use Shanchol without waiting for final registration, says Sack. "This vaccine is safe, cheap, dosed orally, and without side effects."

Stockpile plans

The International Vaccine Institute in Seoul is preparing a report for the WHO that will detail how many cholera vaccines should be stockpiled.

"Had the UN ordered the production of vaccines back in October, there would have been enough for use in high-risk areas," says Sack. "There would not have been a problem of limited supply."

Ideally, says Chao, a stockpile should hold enough vaccines to immunize 50-70% of the high-risk population in an affected country. This could cut deaths by a half. Emergency vaccination plans should be coordinated with rehydration efforts and stockpiling of rehydration kits, Chao adds.

Last December, an expert committee convened by the WHO recommended that a small pilot study be conducted using 250,000-300,000 available doses of Dukoral. But this was not begun because the Haitian government deemed those numbers too small.

This decision was made before the recent election, notes François Lacapère, a PAHO vaccine expert based in Haiti. "In a few weeks, it may be possible to discuss this again," he says.