Contrary to estimates proposed by UN agencies of 400 000 cases of cholera in Haiti this year, a new mathematical modelling study predicts nearly 800 000 cases and over 11 000 deaths*. The findings, published Online First in The Lancet, also suggest that combining the available disease control strategies of improved access to clean water, oral vaccination, and expanded antibiotic use could prevent thousands of deaths.
Importantly, the model also shows that a recent decline in cases of cholera in Haiti is not the result of successful interventions currently being employed, but rather the natural course of the epidemic.
"Although worldwide estimates of the epidemic at present are based on the assumption that the epidemic will attack 4% of the population, this assumption is essentially a guess—based on no data, and ignoring the dynamics of cholera epidemics, such as where people acquire the infection, how they gain immunity, and the role of human interventions such as water allocation or vaccination", explains a US team led by Jason Andrews from Harvard School of Public Health, Boston, USA.
To more accurately predict the spread of cholera in Haiti and aid the allocation of resources, the researchers developed a series of mathematical models taking into account existing disease trends and mechanisms of transmission and immunity.
The model, developed from previous models of cholera transmission and fitted with daily incidence data from each province in Haiti from October 31, 2010 to January 24, 2011, was designed to project the future course of the outbreak and simulate the potential impact of three interventions—clean water, vaccination, and enhanced antibiotic distribution.
The researchers predict 779 000 cases of cholera and 11 100 deaths in Haiti between March 1 and November 30, 2011. However, they also estimate that all three interventions together would prevent 170 000 cases of cholera and 3400 deaths.
According to the model, a 1% reduction in the consumption of contaminated water would prevent 105 000 cases and 1500 deaths, whilst vaccinating 10% of the population is expected to avert 63 000 cases and 900 deaths. Additionally, expanding the early use of antibiotics to all severe cases and half of patients with moderate disease could thwart 900 cases and 1300 deaths.
The authors conclude: "The modelling of cholera transmission is challenging and relatively primitive compared with the modelling of many other infectious diseases…[however] the alternative available to Haiti is a best guess…and might underestimate the resources needed to avert future cases and deaths."
In a Comment, David Sack from John Hopkins Bloomberg School of Public Health, Baltimore, USA, says that the debate about how best to control cholera epidemics has continued for far too long and points out that according to this new model all three interventions are needed to lower rates of cholera and save lives.
He adds: "I hope that cholera will be under control in Haiti within a year. The more realistic expectation is for endemic cholera to continue for many years, as it has in sub-Saharan Africa since 1970, unless a coordinated effort is mounted with all available resources, including improved water and sanitation, improved case management with appropriate antibiotics, and the use of oral vaccines."
Dr Jason Andrews, Massachusetts General Hospital, Harvard Medical School, Boston, USA. T) +1 415 608 9386 E) jandrews6@partners.org
Or Dr Sanjay Basu, University of California, San Francisco, USA.T) +1 415 577 5796 E) sanjay.basu@ucsf.edu
Professor David Sack, John Hopkins Bloomberg School of Public Health, Baltimore, USA. Currently in Bangladesh but contactable by email E) dsack@jhsph.edu
Notes to Editors: *The authors point out that these are projections "under current conditions" and "in the absence of changes in conditions in Haiti." In addition to clean water, vaccines, and antibiotics, many other changes could occur to mitigate this. Their projections are for the plausible scenario that they do not.
Journal
The Lancet