Disease “superspreaders” may have played a major role in the spread of the Ebola virus during the 2014 outbreak in West Africa, according to a new statistical analysis published in Proceedings of the National Academy of Sciences.
An estimated 3 percent of people infected with the virus, which causes diarrhea, vomiting and bleeding, managed to amplify the disease’s spread, passing it to 61 percent of the population in a study that focused on the region of Sierra Leone’s capital, Freetown.
These superspreaders tended to be either younger than 15 or older than 45, though the researchers didn’t study why age might explain a difference in the spread of the virus.
Learning more about how to identify the characteristics of superspreaders, as well as superspreading events like funeral and burials, is a vital part of virus research and could help scientists mute the effect of the next infectious disease outbreak, said study co-author Benjamin Dalziel, an assistant professor of Integrative Biology and Mathematics at Oregon State University.
“Evidence suggests that at the beginning of an emerging epidemic like Ebola, it’s worth asking the question, ‘Where are the superspreading contexts likely to be for this epidemic, and can we identify them?’” Dalziel said. “And if you ask that question early, we [could] put ourselves in the best position to target interventions toward the superspreaders.”
Superspreaders were crucial in prolonging the outbreak
To see how superspreaders fueled the Ebola outbreak, Dalziel analyzed Red Cross data on the Ebola deaths of 200 people near Freetown. These deaths occurred in the community, an understudied population compared with deaths that happened in clinics, and the data included location, time of symptom onset, date of burial, age and sex.
Using mathematical models, Dalziel and co-author, Max Lau of Princeton University tried to create a web of transmission that helped them visualize who acquired the infection from whom.
They found that 3 percent of their population sample had spread the disease to about 61 percent of the rest of the people in the study. They also found that superspreading occurred later on in the epidemic, about 100 days after the outbreak’s peak, suggesting that superspreaders were crucial to prolonging the epidemic’s growth and duration.
Had the superspreaders been identified and quarantined early, Dalziel and Lau write in the study, up to 61 percent of the analyzed fatal infections could have been prevented.
It’s unclear how these results are representative of other regions in the country, or in Liberia and Guinea, which were also hit hard by the epidemic. But Dalziel guessed that, if given the data, he would find that superspreaders also played a similar role in other parts of West Africa.
Superspreading is probably a social, not biological, phenomenon
Dr. Christopher Gill, an associate professor of international health at Boston University’s Center for Global Health and Development, wasn’t involved in the study but said it shed light on an important aspect of the Ebola outbreak.
“[Ebola virus] is an interesting example because it is not particularly infectious as infectious diseases go,” he said. “But clearly Ebola did propogate, and that’s because a small number (3 percent) were responsibly for a very large number of transmissions.”
Based on the most recent outbreak, Ebola is thought to have a basic reproductive rate of 2.4, which means that an infected person will, on average, pass the disease on to 2.4 people (measles, on the other hand, has a reproductive rate between 12 to 18, probably because it can spread through the air).
However, Ebola’s reproductive rate is only an average, and it obscures the very different rates of secondary infection that most Ebola patients had, Gill explained. For instance, Dalziel found that most of the people who got Ebola went on to give the disease to an average of less than one person, which means the epidemic should have died out on its own quickly. But a very small number of Ebola patients were responsible for a large number of transmissions, driving the average up.
Gill was able to make some educated guesses about the superspreaders based on age: One possible explanation could be that the young and the old may experience the virus in a way that makes them more contagious to others or prolongs their infectious period. But because the immune systems of children and older adults are quite different from each other, Gill suspects that the reasons may be social, not biological.
“Children are fussed over when sick, and often seen by many people who worry about them, bring them food, hold them, soothe them, caress them and maybe even kiss them lovingly,” he said. “Similarly, those older than 45 are parents or grandparents, and they too get fussed over and loved and attended to, allowing for lots of infectious exposure to the loving family members.”
Young adults, on the other hand, may be living independently or don’t have wide social networks to care for them when they are sick.
“No matter how one parses the data, the implications should bring a tear to your eye,” Gill concluded. “It is just heartbreaking to think of it this way.”
How superspreading has affected past outbreaks
Scientists have identified superspreaders in other recent outbreaks, like MERS (Middle Eastern respiratory syndrome) and SARS (severe acute respiratory syndrome).
For instance, scientists suspect that superspreaders were responsible for about 71 percent and 75 percent of SARS infections in Hong Kong and Singapore, respectively, during the the 2002-2003 outbreak in Asia. These superspreaders were either people with a lot of close social contacts, who passed the infection on to eight or more people, or people who went to multiple hospitals seeking help.
Scientists analyzing the 2015 MERS outbreak in South Korea also found that five people were responsible for causing 83 percent of the cases in the outbreak. This was mostly due to the hundreds of people they came into contact with when they were treated at the hospital, highlighting the importance of infectious disease control at medical centers.
While some obvious changes can be made at an institutional level, like installing infectious disease wards and training medical staff on infectious disease protocol, scientists still have a long way to go when it comes to identifying superspreaders in the community before they infect many people, said Dalziel.
“We’re not at the point of being able to identify, a priori, what the superspreading contexts are going to be, but we can at least ask the question at the beginning of an epidemic and make that our goal,” he concluded.