Ring and community vaccination for Bundibugyo ebolavirus outbreak response: a stochastic network modelling study
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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.
Funding
IIB is funded by the Canadian Institutes of Health Research (PJT-203753).
Research in context
Evidence before this study
We searched PubMed, medRxiv, WHO guidance, and outbreak-response reports as of July 6, 2026 for studies and reports of rVSV-ZEBOV vaccination, ring vaccination, Bundibugyo ebolavirus, Ebola virus disease transmission models, post-exposure vaccination, and vaccination during Democratic Republic of the Congo Ebola responses. Published evidence supports ring vaccination against Zaire ebolavirus, but there is limited direct evidence for rVSV-ZEBOV effectiveness against Bundibugyo ebolavirus, and limited quantitative evidence comparing reactive ring vaccination with broader community vaccination in the setting of delayed case detection and incomplete contact tracing.
Added value of this study
This study integrates contemporary 2026 Bundibugyo ebolavirus outbreak data with a stochastic household-community network model to quantify the separate and combined effects of case finding, contact tracing, reactive ring vaccination, community vaccination, timing, vaccine coverage, immune-onset kinetics, and behavioural risk compensation. Results demonstrate that strengthening case detection and contact tracing can avert a large fraction of mortality even without vaccination, and that reactive ring vaccination provides only modest additional benefit once these operations are strong. In contrast, rapid community vaccination in affected areas can produce substantially larger mortality reductions by reaching susceptible people before exposure, but its impact is highly sensitive to implementation delay, coverage, vaccine effectiveness, and behavioural assumptions.
Implications of all the available evidence
For Bundibugyo ebolavirus outbreaks, vaccination strategy should be framed as part of a layered response. Case finding and contact tracing remain essential for early detection, isolation, and quarantine, but the largest vaccine-attributable effects are expected when vaccination is deployed rapidly enough and broadly enough to create community immunity before exposure. Reactive ring vaccination remains operationally valuable but should not be assumed to substitute for timely community-level protection when cross-protection is incomplete.
