Local, national, and regional viral haemorrhagic fever pandemic potential in Africa: a multistage analysis

Abstract

Background: Predicting when and where pathogens will emerge is difficult, yet, as shown by the recent Ebola and Zika epidemics, effective and timely responses are key. It is therefore crucial to transition from reactive to proactive responses for these pathogens. To better identify priorities for outbreak mitigation and prevention, we developed a cohesive framework combining disparate methods and data sources, and assessed subnational pandemic potential for four viral haemorrhagic fevers in Africa, Crimean-Congo haemorrhagic fever, Ebola virus disease, Lassa fever, and Marburg virus disease.

Methods: In this multistage analysis, we quantified three stages underlying the potential of widespread viral haemorrhagic fever epidemics. Environmental suitability maps were used to define stage 1, index-case potential, which assesses populations at risk of infection due to spillover from zoonotic hosts or vectors, identifying where index cases could present. Stage 2, outbreak potential, iterates upon an existing framework, the Index for Risk Management, to measure potential for secondary spread in people within specific communities. For stage 3, epidemic potential, we combined local and international scale connectivity assessments with stage 2 to evaluate possible spread of local outbreaks nationally, regionally, and internationally.

Findings: We found epidemic potential to vary within Africa, with regions where viral haemorrhagic fever outbreaks have previously occurred (eg, western Africa) and areas currently considered non-endemic (eg, Cameroon and Ethiopia) both ranking highly. Tracking transitions between stages showed how an index case can escalate into a widespread epidemic in the absence of intervention (eg, Nigeria and Guinea). Our analysis showed Chad, Somalia, and South Sudan to be highly susceptible to any outbreak at subnational levels.

Interpretation: Our analysis provides a unified assessment of potential epidemic trajectories, with the aim of allowing national and international agencies to pre-emptively evaluate needs and target resources. Within each country, our framework identifies at-risk subnational locations in which to improve surveillance, diagnostic capabilities, and health systems in parallel with the design of policies for optimal responses at each stage. In conjunction with pandemic preparedness activities, assessments such as ours can identify regions where needs and provisions do not align, and thus should be targeted for future strengthening and support.

Funding: Paul G Allen Family Foundation, Bill & Melinda Gates Foundation, Wellcome Trust, UK Department for International Development.

Figure 1

Conceptual progression of a viral haemorrhagic fever from animal reservoir to global pandemic Keys stages in the progression to a potential widespread epidemic are summarised. Stage 1, index-case potential, refers to spillover viral transmission from animal reservoir to index cases. Stage 2, outbreak potential, represents an index case infecting individuals within the local community or in a care-giving setting quantified via a composite indicator assessing outbreak receptivity. Stage 3, epidemic potential, reflects the widespread transmission of the virus both at regional and international scales.

Figure 2

Pandemic potential of four African viral haemorrhagic fevers Each column represents the various stages of a potential pandemic, from initial index-case potential (first row) and outbreak potential (second row) to local epidemic potential (third row) and global epidemic potential (fourth row). Columns, moving from left to right, show this progression for Crimean–Congo haemorrhagic fever, Ebola virus disease, Lassa fever, and Marburg virus disease. For each figure, administrative units coloured in red are those with median values (based on 1000 draws) that rank in the top quintile of ranked units; units in dark green have median values that rank in the lowest quintile. Interactive maps are available via the online visualisation tools.

Figure 3

Index-case potential across countries that did not previously report spillover events Stage 1 index-case potential masked by previous reporting of index cases for Crimean–Congo haemorrhagic fever (A), Ebola virus disease (B), Lassa fever (C), and Marburg virus disease (D). Countries in dark grey are those that have previously seen spillover index cases reported. The remaining at-risk administrative units coloured in red are those with median values (based on 1000 draws) that rank in the top quintile of ranked remaining units; those coloured in green have median values that rank in the lowest quintile.

Figure 4

Outbreak receptivity The map displays the final outbreak receptivity indicator, a component in the stage 2 evaluation. Administrative units coloured in red are those with median values (based on 1000 draws) that rank in the top quintile of ranked units; those in dark blue have median values that rank in the lowest quintile. Interactive maps are available via the online visualisation tools.