They are called 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 Sciences.
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 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 percent of the people infected were ultimately 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,” said Benjamin Dalziel, an assistant professor of population biology at Oregon State University and a co-author of the study.
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, Dalziel said.
“Superspreading was more important in driving the epidemic than we realized,” he said.