An analysis suggests that almost two-thirds or 61% of cases in the world’s largest Ebola outbreak were caused by only 3% of infected people who were likely to have been “super-spreaders”. More than 28,600 people were infected with Ebola during the 2014-15 outbreak in West Africa and around 11,300 people died.
Researchers hope that further understanding their role in spreading the infection will help contain the next outbreak.
The study looked at cases in and around Freetown, the capital of Sierra Leone. The researchers were able to tell how many people each infected person was passing the deadly virus onto by looking at the pattern of where and when cases emerged.
Prof. Steven Riley, one of the researchers at Imperial College London, said that most cases had a relatively short infectious period and only generated low numbers of secondary infections, whereas a small number had longer infectious periods and generated more infections. The findings reflect an accurate description of what happened during the outbreak, he added.
Children under 15 and adults over 45 were more likely to be spreading the virus. Riley believes that the spread may be explained by human behavior, whether it has something to do with people coming to care for the young or old.
Super-spreaders have been implicated in other outbreaks, including severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS).
They seem to be a hallmark of emerging infections that are jumping from animal to human hosts. The knowledge could help contain future outbreaks by targeting resources at the super-spreaders.
Huge efforts went into contact-tracing during the Ebola outbreak, which could be focused on super-spreaders in the future. The study may also feed into plans to prepare a stockpile of Ebola vaccine.
According to Prof. Jonathan Ball, a virologist at the University of Nottingham, the recent West African outbreak was on an unprecedented scale and many cases, especially those occurring out in the community, appear to have arisen from a surprisingly small number of infected individuals.
Ball added that knowing who is most likely to transmit the virus can help focus interventions designed to prevent virus spread, and the current study suggests that infected children and the elderly were more likely to pass their virus on.
“Whether this was this due to biological or social factors is unclear, and these will be important questions to address if we are to understand how Ebola virus super-spread occurs,” he said.
The research was a collaboration between Princeton University and Oregon State University in the US, the London School of Hygiene and Tropical Medicine, the International Federation of Red Cross and Red Crescent Societies, Imperial College London, and the US National Institutes of Health.