How Does Health-Zone-Specific Transmission Shape the Potential Impact of Ring Vaccination During the 2026 Bundibugyo Virus Disease Outbreak in the Democratic Republic of the Congo? A Health-Zone-Stratified Branching-Process Modelling Study

Objectives: To determine whether Democratic Republic of the Congo (DRC) health zones with the same operational-priority score required different effective ring reach to achieve the same simulated benefit, and how surveillance reconstruction and modelling assumptions affected estimates. Effective ring reach was the proportion of eligible contacts and contacts of contacts identified, reached, accepting vaccination and vaccinated in time. Design: Health-zone-stratified preparedness modelling using routine aggregate outbreak data. Current transmission (Rt) was estimated from reconstructed reporting intervals, and paired branching-process simulations compared hypothetical ring vaccination with no vaccination. Setting: Five affected DRC provinces; data lock 24 July 2026. Units of analysis: Forty-nine affected health zones identified from 64 official situation reports. Interventions: Scenarios varied vaccine efficacy across five levels, operational timing across three scenarios, additional transmission reduction from non-vaccine measures across four levels, and effective ring reach from 5% to 100%. Main outcome measures: Current Rt; C*25, the lowest effective ring reach producing at least a 25% mean case reduction over 42 days; and C*50, defined similarly for a 50% reduction. Results: Twenty-one health zones met minimum data requirements and 13 had a usable current Rt, producing 18 simulated states because five zones required two plausible reconstructions. Among health zones with priority scoring 14/16, C*25 was 64% in Bunia, 71%-74% in Mongbwalu and 83% in Rwampara. Among zones scoring 11/16, it was 55%-56% in Nizi, 57% in Bambu, 60% in Katwa and 62% in Butembo. Nyankunde did not achieve the 25% target at 100% reach, and no state achieved 50%. Using WHO infection-timing assumptions did not usually change whether transmission was increasing or decreasing, but it made the vaccination target harder to achieve. The model’s short-term predictions were also less reliable than expected. Conclusions: Operational priority and modelled vaccination opportunity differed across health zones. Even zones with the same priority score could require different levels of effective ring reach to achieve the same simulated benefit. These findings are most useful for comparing preparedness demands across zones and scenarios when interpreted alongside recent transmission, surveillance quality and field constraints. They should not be used as forecasts, allocation rules or vaccination-coverage targets.