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

The analysis of matched exposures provides information on the transmission network underpinning the Ebola epidemic (Fig 2) . There was evidence of modest assortativity in exposure patterns by sex (see Table i in S1 Text) and country-specific patterns by age (see Table j in S1 Text). The most important statistic characterising the transmission network is the out-degree distribution, the number of times each person was named as a contact by other Ebola cases. The observed out-degree distribution was best fitted by a logarithmic probability distribution for both funeral and non-funeral contacts (Fig 2A; see sections 1.10 and 2.8 in S1 Text). Since the network is not known in its entirety, but only through a sample of cases and their matched contacts, additional assumptions were needed to infer the true offspring distribution (the distribution of the number of secondary cases infected by each case) from the observed outdegree distribution. Under the assumption that the matched exposures are representative of the underlying transmission network, we find high to extreme variability in the offspring distribution ( Fig 2B; see section 1.10 in S1 Text). The estimated coefficient of variation for the offspring distribution ranges from 1.6 to 5.6 depending on assumptions (see section 1.10 in S1 Text). This implies that 5% of cases accounted for at least 30% of all new infections and that 20% of cases accounted for at least 73% of new infections (Fig 2B; see Figure q in S1 Text), a phenomenon termed super-spreading [26] . Super-spreading was found to affect both nonfuneral and funeral transmissions equally (see Tables f and g in S1 Text).