Between 2013 and 2016, tens of thousands of people in West Africa contracted Ebola virus disease, making the epidemic the largest and longest on record. But the number of reported cases — roughly 28,000 — may be an underestimate of the total number of people infected, since it includes only people who showed symptoms of disease. Researchers reporting in PLOS Neglected Tropical Diseases have discovered individuals in Sierra Leone who showed no signs of the disease but have evidence of prior Ebola infection in their immune systems.


An estimated 10-80 percent of people who show symptoms of Ebola virus disease (EVD) die, depending on their access to intensive care. However, researchers have also seen some evidence that a minority of people may catch the Ebola virus without developing symptoms. During the 2013-2016 outbreak, however, these so-called “minimally symptomatic infections” were not considered epidemiologically relevant to models, projections, or intervention efforts so were not studied or tracked in detail.

From October 2015 through January 2016, a year after the peak of the Ebola epidemic, Eugene Richardson, of Partners In Health, and colleagues conducted a survey of people living in Sukudu, Kono District, Sierra Leone, a village of 900 inhabitants which had been a major “hotspot” of Ebola. The team identified 193 adults and children over age 4 who had lived with or shared a latrine with a confirmed case of Ebola during the period of active transmission but were never identified as EVD cases. The researchers collected blood from 187 of these individuals. They then tested the blood samples — as well as positive and negative controls from known Ebola cases and people not exposed to the virus — for Ebola glycoprotein antibodies, the presence of which indicates a past Ebola infection.

Of the 187 exposed individuals, none of whom were previously known to have Ebola virus disease, the researchers identified 14 who tested positive for Ebola antibodies. Two of these 14 admitted having a fever while being quarantined, while the other 12 denied any signs or symptoms. However, the study could not pinpoint the timing of any past infection, nor guarantee that any individuals were not prevaricating about a lack of symptoms. The results also can’t be extrapolated to other villages affected by the outbreak without more data.

“The findings provide further evidence that Ebola, like many other viral infections, presents with a spectrum of clinical manifestations, including minimally symptomatic infection,” the researchers write. “These data also suggest that a significant portion of Ebola transmission events may have gone undetected during the outbreak.”