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, 23.3%-72.6%]). 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; asymptomatic infection; household; secondary attack rate.

Figure 1.

Forest plot: overall estimate. The denominator is the number of exposed household contacts of infectious Ebola virus disease cases. The numerator is the number of these contacts who develop disease. Abbreviations: DRC, Democratic Republic of the Congo; RE, random-effects; SAR, secondary attack rate; WHO, world health organization.

Figure 2.

Forest plot: direct contact. Abbreviations: DRC, Democratic Republic of the Congo; RE, random-effects; SAR, secondary attack rate; WHO, world health organization.

Figure 3.

Forest plot: no direct contact. Abbreviations: DRC, Democratic Republic of the Congo; RE, random-effects; SAR, secondary attack rate.

Figure 4.

Forest plot: nursing care. For Reaves et al [18], the authors assumed that a high-risk contact was equivalent to providing nursing care as it is defined as “percutaneous or mucous membrane exposure to, or direct skin contact with blood or body fluids of an Ebola patient or a corpse  …  without appropriate personal protective equipment.” Abbreviations: RE, random-effects; SAR, secondary attack rate; WHO, world health organization.

Figure 5.

Forest plot: direct contact but no nursing care provided. Abbreviations: RE, random-effects; SAR, secondary attack rate.

Figure 6.

Forest plot: generation of transmission. Abbreviations: SAR, secondary attack rate; WHO, world health organization.

Figure 7.

Forest plot: asymptomatic proportion. Abbreviations: DRC, Democratic Republic of the Congo; Pt. Est., point estimate; RE, random-effects.