Transmissibility and Pathogenicity of Ebola Virus: A Systematic Review and Meta-analysis of Household Secondary Attack Rate and Asymptomatic Infection

Abstract

Factors affecting our ability to control an Ebola outbreak include transmissibility of the virus and the proportion of transmissions occurring asymptomatically. We performed a meta-analysis of Ebola household secondary attack rate (SAR), disaggregating by type of exposure (direct contact, no direct contact, nursing care, direct contact but no nursing care). The estimated overall household SAR is 12.5% (95% confidence interval [CI], 8.6%-16.3%). Transmission was driven by direct contact, with little transmission occurring in its absence (SAR, 0.8% [95% CI, 0%-2.3%]). The greatest risk factor was the provision of nursing care (SAR, 47.9% [95% CI,.6%]). There was evidence of a decline in household SAR for direct contact between 1976 and 2014 (P = .018). We estimate that 27.1% (95% CI, 14.5%-39.6%) of Ebola infections are asymptomatic. Our findings suggest that surveillance and containment measures should be effective for controlling Ebola.Keywords. Ebola; household; secondary attack rate; asymptomatic infection.For emerging infectious agents, such as Ebola virus (EV), effective prophylactic agents, therapeutics, and vaccines have not been available. The preferred strategy of surveillance and containment is achieved through isolation of cases, intense contact tracing, and active monitoring. Such an approach has historically been effective for ending Ebola outbreaks. In contrast, the 2013-2016 outbreak in West Africa was larger than all prior outbreaks combined [1].Fraser et al propose a framework where the key elements that impact our ability to contain an outbreak are (1) the disease generation time, which is the mean time between infection of an individual and infection of secondary cases, (2) the transmissibility of the virus, and (3) the proportion of asymptomatic transmissions [2]. In addition, (4) availability of prophylactic agents, therapeutics, and vaccines is key. To understand the first 2 elements, household studies are especially useful as contacts are clearly defined and remain fairly constant across cultural settings.Very little is known about the impact of asymptomatic infection on Ebola outbreaks, including the level of pathogenicity of EV, defined as the proportion of Ebola infections that are symptomatic [3]. The proportion of transmissions occurring asymptomatically has an important bearing on our ability to contain an outbreak, with containment measures being less effective if asymptomatic individuals are infectious.In this article, we summarize the transmissibility and pathogenicity of EV. We present a meta-analytic summary of transmission within households, disaggregated by type of exposure. We also present estimates of the asymptomatic proportion of the virus from serosurveys. Most of the data described are from earlier outbreaks, as very few data are available on the epidemic in West Africa. METHODSTransmissibility is measured by the household secondary attack rate (SAR). The SAR is the probability that an exposed susceptible person develops disease over the duration of infectiousness in a ca...

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