Ring and community vaccination for Bundibugyo ebolavirus outbreak response: a stochastic network modelling study

Background: Vaccination with rVSV-ZEBOV is highly effective against Zaire ebolavirus, but protection against Bundibugyo ebolavirus (BDBV) is unknown. We used a stochastic network model of the 2026 Democratic Republic of the Congo BDBV outbreak to evaluate the potential impact of a partially cross-protective vaccine under operationally realistic conditions. Methods: We developed a susceptible-exposed-infectious-recovered model on a two-layer household-community contact network calibrated to public data through July 5, 2026. The model incorporated stochastic detection, isolation, first- and second-degree contact tracing, reactive ring vaccination, and community vaccination. Base-case vaccine efficacy was 45% and included post-exposure protection against disease and mortality, with time to protection modelled as a continuous sigmoidal function. Sensitivity analyses varied vaccine efficacy, timing, case detection, and contact tracing. Findings: Increasing case detection from 30% to 70% and contact tracing from 30% to 80% reduced deaths by 59.8% (IQR 54.6-65.0) compared with base operations without vaccination. Adding reactive ring vaccination reduced deaths by 65.5% (IQR 59.1-71.2) versus base, but by 13.6% (IQR -3.2 to 27.9) versus enhanced operations alone. Community vaccination at 20-80% coverage reduced deaths by 47.1-91.0%. With 50% community coverage, mortality reduction declined from 86% at outbreak declaration to 58% with a 14-day delay. Ring vaccination impact was sensitive to immune-onset timing, with incremental mortality benefit declining from 23% to 10% as the assumed immune-onset midpoint increased from 5 to 14 days. Interpretation: Strengthening case finding and contact tracing is central to mortality reduction in BDBV outbreak response. If rVSV-ZEBOV provides clinically meaningful cross-protection against BDBV, reactive ring vaccination could provide a measurable but modest incremental benefit when operations are already strong. Larger mortality reductions require vaccination that reaches susceptible individuals before exposure, which is more consistent with rapid community vaccination in affected areas.