Ebola Resource Center

Ebola Resource Center

February 22, 2017
2 min read

Small number of ‘superspreaders’ caused most Ebola infections, drove West Africa epidemic

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Health officials could have prevented most cases of the 2014 Ebola virus outbreak by finding and quarantining a tiny percentage of patients, according to a study.

Researchers concluded that about 3% of people who contracted the Ebola virus (EBOV) during the outbreak were responsible for about 61% of infections. Such hosts were deemed “superspreaders” because of their exceptional capacity to pass on the virus that spread throughout Guinea, Sierra Leone and Liberia and killed more than 11,000 people.

“We find that the epidemic was largely driven and sustained by superspreadings that are ubiquitous throughout the outbreak and that age is an important demographic predictor for superspreading,” researchers wrote in Proceedings of the National Academy of Sciences.

“Our results highlight the importance of control measures targeted at potential superspreaders and enhance understanding of causes and consequences of superspreading for EBOV.”

Researchers reviewed community-based data gathered from the International Federation of the Red Cross’ Safe and Dignified Burials program between Oct. 20, 2014, and March 30, 2015, in the Freetown, Sierra Leone, area.

That included the locations at which the bodies of 200 EBOV victims were collected, most of which were the victims’ homes. Researchers sought in part to examine cases that had not been brought to clinics, thereby providing a new, community-based data set.

Using those data and an age-specific framework, they inferred how many other people each host infected to find potential superspreaders.

The researchers estimated the mean infectious period, or time from symptom onset to death or burial, was 3.9 days, and the mean EBOV generation time was 10.9 days for the community-based cases. Other studies had estimated those data at 8 days and 15.3 days, respectively, among cases at clinics.

“These discrepancies potentially highlight systematic differences between community-based cases and cases notified in clinical care systems, with terminal community-based cases progressing significantly more rapidly,” the researchers wrote.

They also found that most secondary cases of EBOV, on average, led to less than one offspring.

“Thus, the epidemic growth appeared to be fueled mostly by only a few superspreaders,” they explained.

The community-based cases may have also helped keep the epidemic going when transmission slowed at clinics.

“Later in the outbreak, most infected individuals were able to get a bed at an Ebola treatment center (ETC) and largely did not further transmit,” the researchers wrote.

“As a result, those superspreaders in the community who did not make it to ETCs may have played an increasingly important role in sustaining the epidemic by generating more secondary cases.”

Victims in the community-based dataset who were aged younger than 15 years and those aged 45 to 60 years had the highest instantaneous transmission hazard levels, the researchers found.

“One plausible explanation, from the social perspective, may be that the young and old are much more likely to have and infect lots of visitors compared to other age groups,” the researchers explained.

“A parallel corollary is that the young and old might be more likely to have others caring for them.”

They suggested further studies may help clinicians find relationships between age and virology.

They added that a new approach may prove more effective in controlling outbreaks, at least those involving superspreading.

“During the … EBOV epidemic, millions of dollars were spent implementing message strategies about Ebola prevention and control across entire countries,” the researchers noted.

“Our results suggest that message strategies targeting individuals with higher risk may be useful to prevent superspreading events and the persistence of the outbreak.”  by Joe Green

Disclosure: The researchers report no relevant financial disclosures.