Introduction and sustained-transmission risk across DRC health zones during the Bundibugyo virus disease outbreak

Technical Summary: Introduction and Sustained-Transmission Risk Across DRC Health Zones During the Bundibugyo Virus Disease Outbreak

Problem Statement
The Democratic Republic of the Congo (DRC) is currently experiencing an ongoing outbreak of Bundibugyo virus disease (BVD) in Ituri province. The response faces significant challenges, including insecurity, high population mobility, limited healthcare capacity, and the absence of a vaccine. As of June 4, confirmed cases have been detected in Nord-Kivu and Sud-Kivu, indicating the spread has extended beyond the initial focus. The core problem addressed is the need for a rapid risk assessment to identify which spatial communities in the DRC are most likely to receive imported cases and, crucially, which are most likely to amplify the outbreak into sustained transmission. This assessment must be conducted despite limited data regarding human mobility, contact patterns, disease transmissibility, and local tracing/isolation capacities.

Methodology
The authors employed a modeling approach based on a method previously developed in Reference 2 to estimate the "sustained-transmission potential" for every health zone (HZ) in the DRC. This metric quantifies the potential for a single introduced case to generate sustained onward transmission within a specific zone.

The model was parameterized using available population and mobility data to characterize spatial transmission. To address data limitations, the authors tested a broad range of assumptions regarding:

• Disease transmissibility.
• Inference of local and between-community contact patterns from available mobility data.

The study generated two distinct rankings for each health zone:

1. Introduction Risk: Estimated from mobility data (Reference 4), representing the likelihood of a case being imported into a zone.
2. Sustained-Transmission Potential: The likelihood that an introduced case will lead to a self-sustaining outbreak.

The authors compared these rankings to identify zones where high introduction risk coincides with high amplification potential. They noted that while absolute risk estimates varied across scenarios, the rankings of health zones were robust across different assumptions regarding transmissibility and contact patterns.

Key Results

• Introduction Risk: As expected, the highest introduction risks were found in health zones adjacent to the reported foci (Ituri, Nord-Kivu, and Haut-Uele). However, non-negligible risks were also identified in distant urban centers, including the Kinshasa area.
• Sustained-Transmission Potential: Unlike introduction risk, high sustained-transmission potential was distributed widely across the country. High-risk zones included Kinshasa, Kasaï-Oriental, Kongo-Central, Tshopo, Nord-Kivu, Haut-Katanga, Sud-Kivu, Kasaï, and Kasaï-Central. Notably, many of these zones are geographically distant from the current outbreak focus.
• Priority Zones (High Risk on Both Dimensions): By cross-referencing the two rankings, the study identified eight specific health zones that combine high introduction risk with high sustained-transmission potential:
  • Kinshasa: Binza Ozone and Binza Meteo.
  • Nord-Kivu: Karisimbi.
  • Sud-Kivu: Kadutu and Ibanda.
  • Tshopo: Makiso-Kisangani, Kabondo, and Mangobo.

Significance and Claims
The paper claims that its primary contribution is the separation of "importation risk" from "amplification risk." The authors argue that traditional risk assessments often focus solely on proximity to the outbreak, assuming that areas near the source are the only ones of concern. This study demonstrates that health zones near the affected area are more likely to receive cases but are not necessarily the most likely to sustain transmission.

Conversely, the study identifies areas of concern farther from the outbreak—specifically in Kinshasa, Kasaï, and Tshopo—that possess high amplification potential. The authors claim this distinction provides robust, outcome-specific guidance for response planning. They assert that preparedness efforts should not focus solely on areas close to the outbreak but must also target high-amplification zones, even if their introduction risk is lower. The paper suggests that response capacity (surveillance, rapid alert investigation, and pre-positioning of resources) should be prioritized in the eight identified health zones that score high on both dimensions. Furthermore, the authors note that preparedness in these urban centers may be more feasible than in parts of eastern DRC, despite heterogeneous healthcare access and insecurity constraints.