Just 3% of people with Ebola were involved in about 61% of transmissions of the disease in the 2014-15 epidemic in West Africa.
These people have been termed 'superspreaders' and were often young people under the age of 15 or people over the age of 45. They were also more likely to have been treated at home rather than in a hospital, according to a paper published in the journal PNAS.
The study also found that superspreaders had the most influence on the spread of Ebola in the tail-end of the epidemic. The researchers speculate that this could be because by this point many patients were being treated in hospitals, and just a few highly transmitting events were happening in the community.
Data from the Safe and Dignified Burials programme run by the International Federation of Red Cross and Red Crescent Societies and collected in Sierra Leone during the epidemic allowed the researchers to begin to draw more fine-scale conclusions about how the disease spread. This suggests that the results are conservative, as many more unofficial burials may have involved increased risk of Ebola transmission.
However, it's not possible to put together a profile of the traits that put someone at high risk of becoming a superspreader, but a few characteristics and risk factors have been identified so far, study author Benjamin Dalziel of Oregon State University in the US told IBTimes UK. This can help direct public health interventions the next time there is an outbreak of Ebola or another disease in a similar context, to help prevent high-risk people from transmitting the disease to many more people.
Superspreaders shouldn't just be thought of as individuals with a particular profile, Dalziel said, but rather as people in a particular context. An event, such as a funeral at which it was reported that the disease was transmitted to 14 women, might also be considered as a superspreader, he said.
"I think that it's events just as much as people. The Sierra Leone case, it really is the event of the funeral [that is the superspreader]."
Behaviours such as washing the body at the funeral are the risk factors that can lead to a large number of secondary cases of Ebola, he said. But characteristics of individual people are part of this context. The higher a person's standing in the community, more people might attend and people would travel from further away, for example.
"It's a mixture of the person and the situation," said Dalziel. "The overarching idea is of a superspreading context. So targeting those contexts is something that might be possible."
A better and more fine-tuned understanding of the factors that contribute to a superspreading event can help direct public health interventions to reduce the risk of secondary transmission in future epidemics.
The causes behind the trends seen in this study need to be identified in order to use it to guide public health interventions, said epidemiologist Rosalind Eggo of the London School of Hygiene and Tropical Medicine.
"For Ebola you'd have to identify why it is that some people infect more than others – if you can understand why," she said.
"We already know there is variation in the number of transmission events for individuals. This study really adds evidence of this effect during the Ebola outbreak. I think it's a novel use of valuable data."
Understanding superspreading is relevant to a range of infectious diseases, such as SARS in 2003 and MERS outbreaks beginning in 2012.
"Next time when there's another epidemic, Ebola or otherwise, the evidence suggests it's worth early on asking the question, what are the superspreaders here and what are the important contexts for superspreading happening. It's about targeting interventions to those contexts," said Dalziel.