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The story is well known among Ebola experts – the traditional healer who died from the virus in Sierra Leone and whose body infected 13 mourners at her funeral, who in turn infected more than 300 other people.
It’s called a “superspreading” event, and a new analysis shows that just 3 percent of patients caused more than 60 percent of all infections in other people during the West African Ebola epidemic.
It’s another piece of evidence showing that certain people are key to spreading disease. If doctors and public health officials can learn more about why and how, perhaps they can learn how to nip epidemics in the bud.
The 2014-2016 Ebola epidemic spread across Guinea, Sierra Leone and Liberia, infecting more than 28,000 people and killing at least 11,000 of them. The extremely deadly virus is spread person to person through infected bodily fluids and, unlike many other diseases, people on the verge of death and dead bodies can be the most dangerous to others.
"We now see the role of superspreaders as larger than initially suspected."
It didn’t take long for doctors to figure out that traditional funeral practices were helping drive the epidemic. The blood, sweat and other fluids of people dying from Ebola can remain infectious for hours or even days, and washing and dressing a body can be a very important part of many funeral traditions.
Helping people adapt these traditional funeral practices helped finally stop the epidemic, but in the meantime, thousands became infected.
"In the recent Ebola outbreak it's now clear that superspreaders were an important component in driving the epidemic," said Benjamin Dalziel of Oregon State University, who helped write the new study.
"We now see the role of superspreaders as larger than initially suspected,” Dalziel added.
Dalziel and colleagues used computer models to calculate just how many other people an Ebola patient infected during the epidemic, a figure known as R0. They found that on average, each Ebola patient infected 2.39 other people.
But it usually wasn’t that even. Many people didn’t infect anyone else at all, while others infected quite a few.
Related: Ebola Outbreak Tip of the Iceberg
“There wasn't a lot of transmission once people reached hospitals and care centers,” Dalziel said.
It was people who stayed home who drove the spread. Many had nowhere to go while others feared the hospital. Early on, Ebola symptoms look like malaria or other tropical diseases, and people did not even suspect Ebola.
“Our results show that superspreaders play a key role in sustaining onward transmission of the epidemic, and they are responsible for a significant proportion (around 61 percent) of the infections.,” Dalziel and colleagues wrote in the Proceedings of the National Academy of Sciences.
“There wasn't a lot of transmission once people reached hospitals and care centers.”
“Our results also suggest age as a key demographic predictor for superspreading.”
The very young and the very old would likely attract many concerned caregivers, who would hold and kiss sick children, or embrace dying elderly loved ones, the team said.
Superspreaders also helped drive the spread of Severe Acute Respiratory Syndrome, or SARS, in 2002 and 2003. SARS infected more than 8,000 people and killed 774 before it was stopped, and researchers were struck by a few superspreading events such as a man who infected 20 others at a Hong Kong hotel, who were then linked to more than 165 other infections in three countries.
A superspreader also carried Middle East Respiratory Syndrome (MERS) virus – including a single South Korean traveler who directly infected 29 people, who went on to infect at least 106 more people.
Ebola was eventually stopped not with vaccines or drugs, but with extensive public health efforts that focused on safe burial and quick hospitalization of people with symptoms.
"As we can learn more about these infection pathways, we should be better able to focus on the types of individual behavior and demographics that are at highest risk for becoming infected, and transmitting infection," Dalziel said.