Cholera continued its relentless march across Yemen in the month of September. The epidemic saw a more than a 650% increase over the summer, from approximately 100,000 total cases in June to more than 750,000 suspected cases and 2,100 deaths as of September 27, according to WHO. The Gulf country is now home to the largest cholera outbreak ever recorded—surpassing the Haiti epidemic between 2010 and 2015.

The epidemic’s ferocious and unexpected spread has raised a number of questions. How did the number of cases grow so large? Why is the case fatality so low? And perhaps most importantly, is the humanitarian response working? It should be noted that the answers to these questions are inherently murky and controversial, given the difficulty in retrieving accurate and credible information in the war-torn country.

Why the high incidence and transmission?

There is no single answer as to why the outbreak spread so quickly and uncontrollably, exceeding many projections. However, the mode of transmission for Vibrio cholerae is unchanged since the waterborne bacterium’s discovery in the 19th century: poor sanitation and a lack of safe drinking water.

Waleed Alhariri, an expert at the Yemen-based Sana’a Center for Strategic Studies and a fellow at Columbia Law School, told Outbreak Observatory that the outbreak is likely “a direct result of [the war] and the breakdown in basic public services—such as water, sewage, and garbage collection—and there is no sign that these services are going to be brought back online any time soon.” According to Alhariri, “The international community needs to...prioritize funding for the humanitarian response and apply diplomatic pressure to allow for greater access for humanitarian aid and staff to the regions in Yemen in greatest need.” Currently, the UN has received only 45% of its humanitarian funding request.

Furthermore, summer weather and rainfall may have exacerbated the epidemic by causing wastewater runoff and contamination of water supplies, with more than half of the country lacking access to safe drinking water. A study analyzing Haiti’s 2010-11 cholera epidemic confirmed the critical role of environmental factors, such as warm air temperature and heavy rainfall, in creating favorable conditions for V. cholerae growth as well as increased exposure to wastewater.

Why the low case fatality?

The cholera outbreak in Yemen raises another question: Why is the case fatality so low even though the number of cases has skyrocketed? WHO guidelines advise that with early and proper treatment, the case fatality for cholera rests below 1%. In other words, fewer than 1 in 100 cases will die from the illness. Yemen witnessed 2,122 deaths out of 753,098 suspected cases as of September 27, an encouraging case fatality of less than 0.3%. This may be surprising given that roughly half of all medical facilities in the country have been destroyed in the war, leaving many victims without access to medical care.

One potential explanation for the low case fatality is that the number of cholera-associated deaths is underreported and/or underestimated. Jamie McGoldrick, the UN Resident and Humanitarian Coordinator in Yemen, appeared to hint at this possibility when he said last Friday:

No one knows how many people have died. No one knows how many people have been killed because of this crisis, because 50% of all health structures that record the actual deaths and injuries are not working. People go to villages and die because there’s no health services for them.

This pattern may be consistent with previous cholera outbreaks. For example, a retrospective survey analysis of the 2010-11 Haiti cholera epidemic found that there was “a substantially higher cholera mortality..than previously reported.” This amounted to a roughly threefold increase in actual fatalities—many of which were initially attributed to acute watery diarrhea—compared to reported fatalities.

Is the humanitarian response effective?

Another potential explanation for the low case fatality is that the humanitarian response is helping. The WHO, UNICEF, and 120 other humanitarian responders are active on the ground in all regions of Yemen, delivering lifesaving humanitarian assistance—including providing oral rehydration solution, chlorinating drinking water, and distributing hygiene kits. In July, the WHO suspended its planned cholera vaccination campaign—the pros and cons of which were discussed in a previous Outbreak Thursday post—stating that a preventive campaign would not be advantageous now that the disease has proliferated widely. As such, the prudent focus on providing immediate cholera treatment and humanitarian relief may be saving thousands of lives in the short-term.

However, experts worry about prospects for long-term recovery. McGoldrick argues that the disease will continue as part of a “vicious cycle” in which it exacerbates the very conditions under which it thrives. Ultimately, rebuilding Yemen’s infrastructure, including fresh water and sanitation systems, and resuming essential services, including healthcare and sanitation services, will be required to prevent a resurgence of the outbreak after it reaches peak incidence later this year.


Outbreak Observatory aims to collect information on challenges and solutions associated with outbreak response and share it broadly in near-real time to allow others to learn from these experiences in order to improve global outbreak response capabilities.