A new study has shown that in the catastrophic 2014-2015 Ebola epidemic in West Africa, about three percent of the people infected were responsible for infecting 61 percent of all cases.
The issue of so-called "superspreaders," according to researchers who published their findings in Proceedings of the National Academy of Sciences, is so significant that it's important to put a better face on just who these people are and then better reach them with public health measures designed to control the spread of infectious disease during epidemics.
The research indicated that Ebola superspreaders often fit into certain age groups and were based more in the community than in health care facilities.
The research was led by Princeton University in collaboration with researchers from Oregon State University (OSU), the London School of Hygiene and Tropical Medicine, the International Federation of Red Cross and Red Crescent Societies, the Imperial College London, and the National Institutes of Health.
In addition, the superspreaders continued to spread the disease after many of the people first infected had been placed in care facilities, where transmission was much better controlled.
So, if superspreading had been completely controlled, almost two thirds of the infections might have been prevented, said the researchers, who noted that their findings were conservative, since they only focused on people who had been buried safely during the 2014 Ebola epidemic.
Superspreaders have been implicated in the spread of severe acute respiratory syndrome, or SARS, in 2003, and the more recent Middle East respiratory syndrome in 2012.
While researchers increasingly appreciate that not all individuals play an equal role in spreading an infectious disease, there is less understanding of who and how important these superspreaders are.
"In the recent Ebola outbreak it's now clear that superspreaders were an important component in driving the epidemic," said Benjamin Dalziel, an assistant professor of population biology in two departments of the College of Science at OSU, and co-author of the study.
"We now see the role of superspreaders as larger than initially suspected. There wasn't a lot of transmission once people reached hospitals and care centers,"Benjamin Dalziel said.
Because case counts during the epidemic relied heavily on hospital data, those hospitalized cases tended to be the cases we 'saw.' However, it was the cases you didn't see that really drove the epidemic, particularly people who died at home, without making it to a care center," Benjamin Dalziel said.
"In our analysis," Dalziel was quoted as saying in a news release from OSU, "we were able to see a web of transmission that would often track back to a community-based superspreader."
Superspreading has been cited in many first-hand narratives of Ebola transmission. The outbreak size of the 2014 Ebola epidemic in Africa was unprecedented, and early control measures failed.
Researchers suggest that messages tailored to individuals with higher risk and certain types of behavior may have been more successful, and prevented the epidemic from being so persistent.