Ebola ‘superspreaders’ accounted for nearly two-thirds of infections, study finds
They are called the superspreaders, the minority of people who are responsible for infecting many others during epidemics of infectious diseases. Perhaps the most famous superspreader was Typhoid Mary, presumed to have infected 51 people, three of whom died, between 1900 and 1907.
Now scientists studying how Ebola spread during the 2014-2015 epidemic in West Africa say superspreaders played a bigger role than was previously known, according to findings published this week in the Proceedings of the National Academy of Science.
If superspreading had been completely controlled, almost two-thirds of the infections might have been prevented, scientists said.
More than 28,000 confirmed, probable and suspected cases of Ebola were reported in West Africa during the outbreak, including more than 11,000 deaths, according to the World Health Organisation.
Researchers at Princeton University and Oregon State University conducted a retrospective analysis of the timing and location of 200 community burials between October 2014 and March 2015 in the urban areas around Freetown, the capital of Sierra Leone.
Using a mathematical model, they reconstructed the transmission network to see what proportion of cases were caused by superspreaders. They estimated that about 3 per cent of the people infected were ultimately responsible for infecting about 61 per cent of cases.
“It’s similar to looking at a blood spatter pattern and figuring out where the shooter was standing,” he said.
Based on the evidence of disease transmission, that pattern - a small number of individuals responsible for the majority of infections - also holds true in Guinea and Liberia, the other two countries hit hardest by Ebola, said Benjamin Dalziel, an assistant professor of population biology at Oregon State University and a co-author of the study.
“Superspreading was more important in dr iving the epidemic than we realised,” he said.
Superspreaders of Ebola tended to be children younger than 15 or adults between 40 and 55 years old, he said. They were based in the community rather than in health-care facilities, and they continued to spread the disease after many of the people first infected were already in treatment centres, where transmission was much better controlled.
Older adults were most probably caring for the children. These caregivers were also more likely to be the people in charge of organising large funerals, he said. Researchers said their findings about the importance of superspreaders were conservative because they focused only on people who had been buried safely.
Ebola spreads primarily through contact with bodily fluids. During the epidemic, superspreaders were cited in numerous news stories about Ebola’s spread. Often, transmission occurred during caregiving at home and during funeral preparations, especially washing and touching the bodies of loved ones.
Researchers said the study provides a new statistical framework that allowed scientists to measure how important superspreaders were in fueling the epidemic. In the future, public health officials should consider asking the question “What are the scenarios where superspreading might occur,” Dalziel said. Just asking that question might help tailor better methods of controlling an outbreak, he said.
Superspreaders have played a role in the spread of several other infectious diseases. During the 2002-2003 outbreak of severe acute respiratory syndrome, or SARS, superspreading was considered one of the most notable features of the outbreak.
In one such event in Hong Kong, a 26-year-old sick man who was admitted to a hospital for treatment infected 156 people, including hospital staff, patients and visitors.
A single superspreader in a busy hospital emergency department also spread MERS, Middle East respiratory syndrome, to 82 people in just three days in May 2015 during an outbreak of the virus in South Korea.