The Magic Number That Could End the Ebola Epidemic

There are a lot of scary numbers floating around about Ebola. Take 1.4 million: the CDC’s worst-case scenario for Ebola cases in Western Africa by the end of January. Or two: the approximate number of healthy people infected by each new Ebola patient.

But perhaps the most important Ebola number right now is 70 percent. That’s the proportion of patients who need to be isolated -- in treatment centers or at least in their homes -- in order to put a quick end to the Ebola outbreak, according to the U.S. Centers for Disease Control and Prevention.

“Once 70 percent of patients are effectively isolated, the outbreak decreases at a rate nearly equal to the initial rate of increase,” researchers wrote today in the CDC’s Morbidity and Mortality Weekly Report. If 70 percent of the current outbreak was achieved by late December, the epidemic “would be almost ended by January 20.”

Seventy percent is a number full of hope and dread. Hope, because it’s a goal that feels attainable; a developed country would be able to handle 70 percent isolation on its own soil in short order. Dread, because in Ebola-swept regions like Liberia and Sierra Leone, we are nowhere near achieving it. Right now, only about 18 percent of Ebola patients in Liberia are being isolated.

Each day the epidemic persists makes 70 percent more difficult to reach. More doctors, hospital beds and treatment centers will be needed, and more people must be educated about the disease. For every 30-day delay, the peak number of new daily cases triples, according to a model of the disease created by the CDC.

In each of the three scenarios modeled in the chart above, 70 percent isolation is eventually reached and the outbreak is brought under control. The difference is how long it takes to initiate major interventions (building and staffing treatment centers, distributing supplies) and and how many lives are lost as a result.

Despite its reputation as a killer, Ebola isn’t very good at reproducing itself. The virus is spread through body fluids, not air, and it often kills patients before they have a chance to spread the disease widely. When 70 percent of patients are isolated, the disease no longer spreads fast enough to replace dying or recovering patients. It burns itself out.

All it takes is to break the epidemic is to reach that magic number. What makes the current outbreak so difficult is that it’s happening in war-impoverished countries that have no prior experience with Ebola and very few doctors and hospitals to start with. The size of the outbreak also puts it in uncharted territory.

The 1.4 million worst-case projection by the CDC, by the agency's own estimation, is “very unlikely.” It doesn’t account for major health interventions, which are already underway. For example, U.S. soldiers have started arriving in Liberia after U.S. President Barack Obama pledged to help build as many as 20 treatment centers, train about 500 health-care providers and send 3,000 troops to assist. The Pentagon may spend as much as $1 billion fighting the disease.

That’s a lot of numbers. Let’s hope they add up to 70.