Ebola control: effect of asymptomatic infection and acquired immunity

Bellan, Steven E.; Pulliam, Juliet R. C.; Dushoff, Jonathan; Meyers, Lauren Ancel

doi:10.1016/s0140-6736(14)61839-0

Cited by 80 publications

(68 citation statements)

References 7 publications

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“…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.…”

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confidence: 99%

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

et al. 2016

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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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confidence: 99%

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

et al. 2016

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“…Although the number of recovered Ebola cases that have been reported in Liberia is far from the herd immunity threshold, a high proportion of asymptomatic infections that result in seroconversion without case detection reduces the pool of susceptibles that could conceivably achieve herd immunity. 10 Our results indicate that asymptomatic infections can lead to some tapering of the epidemic over the current time scale, although accumulation of asymptomatic infections may have a greater impact on transmission at a more localized scale than county-level captured in our model. Nevertheless, we found that even if 99% of Ebola infections are asymptomatic, this factor would not have been sufficient to mediate the dramatic reduction of transmission that occurred.…”

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confidence: 62%

“…6 We conducted scenario analysis to account for potential asymptomatic and underreporting cases suggested by previous studies. [10][11][12] A systematic review was conducted to determine the timing of key intervention events since the beginning of the Ebola outbreak in Liberia. All archived situation reports from the Liberian Ministry of Health and Social Welfare, the United Nations Children's Fund (UNICEF), and the WHO Ebola Response Roadmap were reviewed for documentation of intervention events.…”

Section: Methodsmentioning

confidence: 99%

Retrospective Analysis of the 2014–2015 Ebola Epidemic in Liberia

Atkins

Pandey

Wenzel

et al. 2016

The American Journal of Tropical Medicine and Hygiene

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Abstract. The 2014-2015 Ebola epidemic has been the most protracted and devastating in the history of the disease. To prevent future outbreaks on this scale, it is imperative to understand the reasons that led to eventual disease control. Here, we evaluated the shifts of Ebola dynamics at national and local scales during the epidemic in Liberia. We used a transmission model calibrated to epidemiological data between June 9 and December 31, 2014, to estimate the extent of community and hospital transmission. We found that despite varied local epidemic patterns, community transmission was reduced by 40-80% in all the counties analyzed. Our model suggests that the tapering of the epidemic was achieved through reductions in community transmission, rather than accumulation of immune individuals through asymptomatic infection and unreported cases. Although the times at which this transmission reduction occurred in the majority of the Liberian counties started before any large expansion in hospital capacity and the distribution of home protection kits, it remains difficult to associate the presence of interventions with reductions in Ebola incidence.

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“…Interestingly, the CFR range of 30–90%, depending on the virus species, for most previous outbreaks including the present outbreak,2 suggests that a substantial proportion of exposed people are asymptomatic or do not come down with the disease due to protective immunity to the infection 5. These asymptomatic individuals also contribute to boost herd immunity and therefore could potentially dampen the spread of the epidemic.…”

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confidence: 80%