In 2014 and 2015, Ebola spread through West Africa like wildfire, affecting over 28,000 people in Guinea, Sierra Leone and Liberia and killing 11,310. But just how did the dangerous virus spread? A new study has a surprising answer, reports the BBC’s James Gallagher—the majority of cases were caused by a small minority of infected people.
A new paper published in the journal Proceedings of the National Academy of Sciences suggests that just three percent of people with Ebola were responsible for around 61 percent of cases. The study, which used statistical models to show how the disease was transmitted, found that age was the biggest predictor of whether an individual would spread the virus or not.
Researchers used data from a burial program conducted by the Red Cross that included GPS locations of where the bodies of 200 people who died of Ebola were collected. The data set also included information on their age, sex and time of burial. Using that data, researchers were able to infer how many people each infected person got sick. They found that people below 15 and above 45 years of age were more likely to spread the virus than those in the middle range.
This phenomenon, also known as “superspreading,” has been observed before. In 2015, an outbreak of MERS in South Korea occurred when a single patient infected at least 22 other people. And most are probably familiar with the story of Typhoid Mary, a superspreader who was herself immune to typhoid, but infected 51 people in a short period of time. Mary Mallon was then placed in a forced, decades-long quarantine.
As The Wall Street Journal’s Sumathi Reddy reports, scientists think that 20 percent of the population spreads disease more easily than the other 80 percent. However, the jury’s still out on exactly why. Steven Riley, one of the Ebola paper’s co-authors, tells Gallagher that he thinks the disease's spread was due to human behavior and that perhaps the fact that the young or old were taken care of by people in the middle age bracket.
One thing is clear: Superspreading can make the difference between a blip and a full-blown epidemic. Epidemiologists are getting better at analyzing data to determine who spreads disease. But given the short incubation period of many diseases—Ebola, for example, can incubate in as few as two days—it can be hard to stop contagion before the death toll begins to climb. Though nothing can replace the lives lost in epidemics, perhaps scientists can learn from these deaths to one day stop future outbreaks.