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 2014-15 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 Organization.
Researchers at Princeton University and Oregon State University conducted an 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 percent of the people infected were responsible for infecting about 61 percent 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 driving the epidemic than we realized,” 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 centers, where transmission was better controlled.
Older adults were probably caring for the children. These caregivers were also more likely to be the people in charge of organizing 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.
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. Asking that question might help tailor better methods of controlling an outbreak, he said.