A review of epidemiological parameters from Ebola outbreaks to inform early public health decision-making

Abstract

The unprecedented scale of the Ebola outbreak in West Africa has, as of 29 April 2015, resulted in more than 10,884 deaths among 26,277 cases. Prior to the ongoing outbreak, Ebola virus disease (EVD) caused relatively small outbreaks (maximum outbreak size 425 in Gulu, Uganda) in isolated populations in central Africa. Here, we have compiled a comprehensive database of estimates of epidemiological parameters based on data from past outbreaks, including the incubation period distribution, case fatality rate, basic reproduction number (R 0 ), effective reproduction number (R t ) and delay distributions. We have compared these to parameter estimates from the ongoing outbreak in West Africa. The ongoing outbreak, because of its size, provides a unique opportunity to better understand transmission patterns of EVD. We have not performed a meta-analysis of the data, but rather summarize the estimates by virus from comprehensive investigations of EVD and Marburg outbreaks over the past 40 years. These estimates can be used to parameterize transmission models to improve understanding of initial spread of EVD outbreaks and to inform surveillance and control guidelines.

Figure 1. Estimates of the incubation period distribution by virus, year of outbreak and study.

The dots represent the mean or median estimate and the lines illustrate the range, for all studies, with the exception of MacNeil et al.  and WHO Ebola Response Team (ERT) 2015 (fitted) where the line represents the 95% CI for the estimate. Chowell et al. , Eichner et al. , WHO ERT 2014 (multi-day observed) and Lekone and Finkenstädt provide standard deviation (s.d.).

Figure 2. Overall case fatality rate (CFR) for Ebola by virus and Marburg virus; we include only those outbreaks that have been declared over.

Outbreaks with fewer than 10 fatal patients were excluded from this figure. For each outbreak, the bars represent the rate of fatal (dark color) and recovered (light color) patients. For country specific information see Table 1. The source of each estimate is denoted by the reference number at the top of each bar.