Ebola Outbreak Sustained by Superspreaders
3% of cases cause 61% of infections
Prior to the epic Ebola outbreak of 2014-2015, the prevailing dogma was that the virus was not a pandemic threat because it killed its victims too quickly to sustain itself in a prolonged outbreak. That myth was certainly exploded, and now we have one clue as to why: “superspreaders.”
This infectious disease phenomenon wherein a single case transmits to multiple contacts appeared to play a role in sustaining the Ebola epidemic in West Africa.
“Superspreading is a rather relative concept, its importance is not solely determined by how infectious the virus,” says Max Lau, PhD, lead author of a new study1 and a researcher at Princeton (NJ) University. “When a disease is more fatal and most cases do not further transmit, superspreading events — for example, the “outlier” compared to average infections — are expected to play a more important role relatively.”
Lau and fellow investigators analyzed data and used modeling techniques, concluding that about 3% of cases spread Ebola to 61% of those infected in the outbreak. “Had the superspreaders been identified and quarantined promptly, a majority of the infections could have been prevented,” they concluded.
Superspreading events occurred primarily in the community and broke down into two key age groups: Those under age 15 or over age 45 were more likely to be superspreaders.
One would intuit that factors such as viral titer, travel and contact with others, the immune competence of contacts, and other factors could influence superspreading events.
“Probably a common theme will be that social behavior shapes superspreading,” Lau says. “Many factors such as viral load and contact pattern will influence how superspreading may actually take place. Our study suggests that, for Ebola, the caregiving pattern — for example, a social contact pattern in which the young and the old are more likely to have more people to care for them — may be an important factor, but we cannot exclude other potential factors such as the viral load difference. We do not have relevant data.”
Another plausible explanation is that, from the social perspective, the young and old are much more likely to have more visitors compared to other age groups, the authors conclude.
“Our results show that Ebola exhibited a prominent superspreading pattern shared by SARS and MERS,” they noted.
Indeed, the WHO reported an outbreak of MERS in a hospital in Riyadh, Saudi Arabia, where a single patient infected 24 contacts — 13 patients and 11 healthcare workers.2 Investigators of a 2015 outbreak of MERS in Samsung Medical Center in Seoul, South Korea, recently published a study that revealed one patient exposed hundreds of patients, visitors, and healthcare workers while in the ED between May 27 and May 29.3 MERS infection was confirmed in 33 patients, 41 visitors, and eight healthcare workers.
The global outbreak of SARS began when a Chinese doctor infected more than a dozen other people staying on the ninth floor of the Metropole Hotel in Hong Kong.4 They departed to their home countries with SARS circulating in their blood.
REFERENCES
- Lau MSY, Dalziel BD, Funk S, et al. Spatial and temporal dynamics of superspreading events in the 2014–2015 West Africa Ebola epidemic. PNAS 2017;114(9) 2337–2342.
- WHO. Update and clarification on recent MERS cases reported by the Kingdom of Saudi Arabia. Geneva, Switzerland. 23 June 2016:
http://bit.ly/2azKtF5. - Sun YC, Kang JM, Young EH, et al. MERS-CoV outbreak following a single patient exposure in an emergency room in South Korea: an epidemiological outbreak study. Lancet. Published online July 8, 2016: http://bit.ly/2aS1nvS.
- CDC. Severe Acute Respiratory Syndrome — Singapore, 2003. MMWR 2003;52(18):405-411.
Prior to the epic Ebola outbreak of 2014-2015, the prevailing dogma was that the virus was not a pandemic threat because it killed its victims too quickly to sustain itself in a prolonged outbreak. That myth was certainly exploded, and now we have one clue as to why: “superspreaders.”
Subscribe Now for Access
You have reached your article limit for the month. We hope you found our articles both enjoyable and insightful. For information on new subscriptions, product trials, alternative billing arrangements or group and site discounts please call 800-688-2421. We look forward to having you as a long-term member of the Relias Media community.