Tracking the rise and fall of Ebola in Sierra Leone

Study pins numbers on outbreak’s devastation, interventions that worked
Nurses treat Ebola patients in Sierra Leone
Nurses wearing personal protective equipment treat Ebola patients at the Kenama treatment center run by the Red Cross Society on November 15, 2014 in Sierra Leone. Photo by Francisco Leong / Getty Images

Life is returning to normal in Guinea, Liberia and Sierra Leone, after the worst Ebola outbreak in history took 11,000 lives. International medical teams have packed up their bags and left. The world has shaken off its Ebola jitters, but now scientists at Fred Hutchinson Cancer Research Center and colleagues from China and Sierra Leone are conducting a kind of extended postmortem, trying to figure out what happened.

After a painstaking process collecting and analyzing information sent from nearly every chiefdom in Sierra Leone — districts comparable to U.S. counties — a team of biostatisticians including Fred Hutch’s Dr. Betz Halloran and Dr. Yang Yang of the University of Florida has published a new study that shows how the Ebola virus tore a swath through that country like an invading army.

Their statistical analysis tracking the “transmission dynamics” of the outbreak provides an unprecedented level of detail that could yield clues for how to stop the next outbreak of this frightening disease, said Yang, who is also a Fred Hutch affiliate investigator.

“We think we have a better picture now of the interventions that were successful,” he said.

Their study, following on the heels of a similar study published in 2015 about the outbreak in neighboring Liberia, also put numbers on the success of federal and international interventions to stop Ebola — showing that the virus’ transmission within households dropped from 9 to 2 percent.

The study, published Monday online in the journal Proceedings of the National Academy of Sciences, shows how the epidemic spread after the first few cases appeared on Sierra Leone’s eastern border with Guinea in May 2014. Other research has traced the start of the outbreak there to a smattering of cases linked to a burial of a traditional healer who died in neighboring Guinea. The Hutch study shows in detail how the Sierra Leone outbreak gathered strength as it travelled westward, flaring at sites where secondary roads met major highways and at towns that had medical facilities toward which the sick were fleeing.

By August, a terrifying epidemic was occurring on the west coast in Freetown, Sierra Leone’s largest city; from there it marched back east to the central part of the nation. The virus continued to snake through the country along these two invasion routes, called in the dry language of science “spatial diffusion corridors.”

These same computer-generated maps also document how the epidemic was slowed and contained during the months of October to December 2014, as Sierra Leone government and United Nations interventions took hold. Throughout last year, the maps show, infections gradually melted away, until the World Health Organization declared the Ebola outbreak over in that country on November 7, 2015 — only to have two more cases flare up in January. Sierra Leone in the end was the hardest hit of the three nations, with more than 8,700 confirmed cases, and 3,590 deaths.

Dr. Betz Halloran
Biostatistician Dr. Betz Halloran Fred Hutch file

In the absence of a vaccine, simpler interventions stemmed the outbreak

Using charts and metrics that track the intensity of the epidemic in Sierra Leone, the report shows the impact of an UN-designed campaign that was first implemented in October 2014.  

The UN measures included isolation of ill patients, identifying those who had been in recent contact with that patient, and a program of “safe and dignified” burials of Ebola victims. Meanwhile, international partners built a makeshift network of healthcare facilities and diagnostic laboratories.

“These types of interventions, combined together, are the best way to combat these epidemics before we get better weapons, like vaccines,” said Yang.

After those interventions were launched in October 2014, the study found, the average number of new infections transmitted by a given patient over a week dropped 43 percent. That decline grew to a 65 percent drop after December 2014, when the goals of 100 percent case isolation and safe burials were said to be achieved.

In other words, despite an initially slow response, the interventions worked.

Reached on her return from a WHO meeting in France this weekend, Halloran said this latest study benefits from data gathered in Sierra Leone by the Hutch team’s Chinese partners. Coauthors of the PNAS study are affiliated with the Beijing Institute of Microbiology and Epidemiology and Anhui Medical University, in Hefei, China, as well as the Sierra Leone Ministry of Health and Sanitation. “It’s a very detailed analysis of an original data set with information at the chiefdom level,” she said.

The study was able to measure transmissibility of the disease even at the household level, a sophisticated measurement called the “household secondary attack rate.” At the height of the epidemic, that transmission rate was 9 percent — a capacity to spread comparable to that of influenza, a disease that unlike Ebola can be easily transmitted through the air. Ebola is caused by contact with body fluids such as blood, saliva, and sweat. It is far deadlier than flu, killing half of those infected. A major source of Ebola transmission was the traditional washing of bodies for burial and close contact between patients and unprotected health care workers.

After the adoption of UN recommendations for isolation and safe burials, however, the household secondary attack rate dropped from 9 to less than 2 percent. With no vaccine and no effective treatment, the frightening Ebola virus could nevertheless be stopped. “These estimates of household transmissibility will help in modeling future epidemics of Ebola and [evaluating] the effectiveness of interventions,” said Halloran.

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Sabin Russell is a staff writer at Fred Hutchinson Cancer Research Center. For two decades he covered medical science, global health, and health care economics for the San Francisco Chronicle, and wrote extensively about infectious diseases, including HIV/AIDS. He was a Knight Science Journalism Fellow at MIT and a freelance writer for the New York Times and Health Affairs. Reach him at

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