Exposure Patterns Driving Ebola Transmission in West Africa: A Retrospective Observational Study

Agua-Agum, Junerlyn; Ariyarajah, Archchun; Aylward, Bruce; Bawo, Luke; Bilivogui, Pepe; Blake, Isobel M.; Brennan, Richard J.; Cawthorne, Amy; Cleary, Eilish; Clement, Peter; Conteh, Roland; Cori, Anne; Dafae, Foday; Dahl, Benjamin; Dangou, Jean-Marie; Diallo, Boubacar; Donnelly, Christl A.; Dorigatti, Ilaria; Dye, Christopher; Eckmanns, Tim; Fallah, Mosoka; Ferguson, Neil M.; Fiebig, Lena; Fraser, Christophe; Garske, Tini; Gonzalez, Lice; Hamblion, Esther; Hamid, Nuha; Hersey, Sara; Hinsley, Wes; Jambei, Amara; Jombart, Thibaut; Kargbo, David; Keita, Sakoba; Kinzer, Michael; George, Fred Kuti; Godefroy, Beatrice; Gutierrez, Giovanna; Kannangarage, Niluka; Mills, Harriet L.; Moller, Thomas; Meijers, Sascha; Mohamed, Yasmine; Morgan, Oliver; Nedjati-Gilani, Gemma; Newton, Emily; Nouvellet, Pierre; Nyenswah, Tolbert; Perea, William; Perkins, Devin; Riley, Steven; Rodier, Guenael; Rondy, Marc; Sagrado, Maria; Savulescu, Camelia; Schafer, Ilana J.; Schumacher, Dirk; Seyler, Thomas; Shah, Anita; Van Kerkhove, Maria D.; Wesseh, C. Samford; Yoti, Zabulon

One key potential determinant of the risk of onward transmission is the stage of progression of clinical illness. The risk of transmission was found to increase over time since symptom onset (Fig 3A) , peaking 2 days after onset, with some exposures estimated to have occurred more than 2 weeks after onset. Our model estimates a small probability of transmission before symptom onset; however, we do not regard this as strong evidence for pre-symptomatic transmission: all reported dates are prone to recall bias, but it is likely that the date of symptom onset is more uncertain than dates of hospitalisation and death, as it is subjective, so individuals may interpret symptom onset differently. Transmission events from non-funeral exposures were estimated to be strongly peaked on the day of and the day after the death of the contact (Fig 3B) . In all, 44% of non-funeral Not all cases who reported funeral exposure explicitly reported whether they had touched the corpse. Cases who reported non-funeral exposure could report multiple types of exposure: belongings-"touched or shared the linens, clothes, or dishes/eating utensils of the case [contact]"; bodily fluids -"touched the body fluids of the case (blood, vomit, saliva, urine, feces)"; in same household-"slept, ate, or spent time in the same household or room as the case"; direct physical-"had direct physical contact with the body of the case". Relationship was not reported for every exposure. We grouped reported relationships into classes: "close family" is defined as siblings, marital, and parent-child relationships; other family members are considered "extended family"; "neighbour" is defined as tenants, lodgers, landlords, and neighbours; "health care" is defined as HCW-patient relationships and caregivers, or any reference to a patient; "other" includes traditional healers, contacts through religious practice, and transport contacts. Type of exposure and relationship type are illustrated graphically in Figure d in S1 Text. We sought to determine whether date of death or date of symptom onset was the stronger predictor of transmission risk. We found that date of death was the stronger predictor, indicating that the timing of death is the strongest determinant of when transmission occurs (see Figure o in S1 Text).