Key data for outbreak evaluation: building on the Ebola experience

Cori, Anne; Donnelly, Christl A.; Dorigatti, Ilaria; Ferguson, Neil M.; Fraser, Christophe; Garske, Tini; Jombart, Thibaut; Nedjati-Gilani, Gemma; Nouvellet, Pierre; Riley, Steven; Van Kerkhove, Maria D.; Mills, Harriet L.; Blake, Isobel M.

Heterogeneity in the number of secondary cases generated by each infected individual affects epidemic establishment and the ease of control. Greater heterogeneity reduces the chance of an outbreak emerging from a single case [50] . However, this heterogeneity can make an established outbreak hard to control using mass interventions, as a single uncontrolled case can generate a large number of secondary cases [50] . Conversely, heterogeneity in transmission may provide opportunities for targeting interventions if the individuals who contribute more to transmission (because of environmental, behavioural and/or biological factors [51] [52] [53] ) share socio-demographic or geographical characteristics that can be identified [50, 54] . Reconstruction of transmission trees (who infects whom) can provide an understanding of who contributes more to transmission. This can be done using detailed case investigations and/or using genetic data [55] [56] [57] [58] . Environmental, behavioural and biological factors may also lead to groups of individuals being disproportionately more likely to acquire infection (e.g. children during influenza outbreaks [53, 59] or health-care workers (HCWs) during Ebola outbreaks [60, 61] ). To identify whether such groups exist and target them appropriately, the proportion infected in each group must be estimated. This requires population size estimates for the different groups, which may be difficult to obtain, as we highlight §3c.