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Background Healthcare and other front-line workers are at particular risk of infection with Ebola virus (EBOV). Despite the large-scale deployment of international responders, few cases of Ebola virus disease have been diagnosed in this group. Since asymptomatic or pauci-symptomatic infection has been described, it is plausible that infections have occurred in healthcare workers but have escaped being diagnosed. We aimed to assess the prevalence of asymptomatic or pauci-symptomatic infection, and of exposure events, among returned responders to the West African Ebola epidemic 2014–2016. Methods and findings We used snowball sampling to identify responders who had returned to the UK or Ireland, and used an online consent and questionnaire to determine their exposure to EBOV and their experience of illness. Oral fluid collection devices were sent and returned by post, and samples were tested using an EBOV IgG capture assay that detects IgG to Ebola glycoprotein. Blood was collected from returnees with reactive samples for further testing. Unexposed UK controls were also recruited. In all, 300 individuals consented, of whom 268 (89.3%) returned an oral fluid sample (OFS). The majority had worked in Sierra Leone in clinical, laboratory, research, and other roles. Fifty-three UK controls consented and provided samples using the same method. Of the returnees, 47 (17.5%) reported that they had had a possible EBOV exposure. Based on their free-text descriptions, using a published risk assessment method, we classified 43 (16%) as having had incidents with risk of Ebola transmission, including five intermediate-risk and one high-risk exposure. Of the returnees, 57 (21%) reported a febrile or diarrhoeal illness in West Africa or within 1 mo of return, of whom 40 (70%) were not tested at the time for EBOV infection. Of the 268 OFSs, 266 were unreactive. Two returnees, who did not experience an illness in West Africa or on return, had OFSs that were reactive on the EBOV IgG capture assay, with similar results on plasma. One individual had no further positive test results; the other had a positive result on a double-antigen bridging assay but not on a competitive assay or on an indirect EBOV IgG ELISA. All 53 controls had non-reactive OFSs. While the participants were not a random sample of returnees, the number participating was high. Conclusions This is the first study, to our knowledge, of the prevalence of EBOV infection in international responders. More than 99% had clear negative results. Sera from two individuals had discordant results on the different assays; both were negative on the competitive assay, suggesting that prior infection was unlikely. The finding that a significant proportion experienced “near miss” exposure events, and that most of those who experienced symptoms did not get tested for EBOV at the time, suggests a need to review and standardise protocols for the management of possible exposure to EBOV, and for the management of illness, across organisations that deploy staff to outbreaks., In a cross-sectional study, Catherine Frances Houlihan and colleagues surveyed international responders to the West African Ebola crisis, after they returned to the UK or Ireland, to analyze their experience of risk., Author summary Why was this study done? A large number of international volunteers worked in West Africa during the largest Ebola outbreak ever seen, but only a small number of international staff were diagnosed with Ebola virus disease. Since asymptomatic or unrecognised infections can occur, it is possible that some infections in international responders to the epidemic were missed. What did the researchers do and find? We carried out an online survey and antibody testing of oral fluid samples from 268 individuals who returned to the UK and Ireland after working in West Africa during the Ebola outbreak. A high number of near miss events were reported, and less than a third of individuals who experienced illness whilst in West Africa or soon after return were tested for Ebola virus while they were unwell. Of the 268 who were tested for antibodies, two had reactive test results, but these were not reactive on further testing. What do these findings mean? Although our study could not reach every individual who returned to the UK after working in West Africa during the Ebola outbreak, we demonstrated that asymptomatic or pauci-symptomatic Ebola virus infection was rare in international healthcare and other workers in our study who responded to the epidemic in West Africa. The descriptions of near miss events and the finding that many of those who experienced illness were not tested at the time suggest that protocols for the management of possible exposure to Ebola virus and for the management of illness should be reviewed and standardised across organisations that deploy staff to outbreaks.