Detected case growth and outcome heterogeneity during the 2026 Bundibugyo virus disease outbreak in the Democratic Republic of the Congo and Uganda

The 2026 Bundibugyo virus disease outbreak in the Democratic Republic of the Congo (DRC), with cross-border cases in Uganda, evolved amid mobility, insecurity, delayed detection, laboratory constraints and pressure on case management. We assessed how official aggregate data with documented source provenance could support real-time description of reported case growth, outcome heterogeneity and response constraints. We conducted a retrospective aggregate time-series and subnational ecological analysis of official situation reports and public health updates. The DRC analysis used national, provincial and health-zone data from situation reports 001– 028 covering 14 May–11 June 2026; Uganda was analysed separately through 10 June 2026. Outcomes included epidemic trajectory, growth of reported cumulative confirmed cases, crude observed case-fatality ratios (CFRs) and selected operational indicators. Growth estimates were treated as descriptive measures of reported case accumulation, not direct estimates of transmission intensity. Between 14 May and 11 June 2026, DRC recorded 689 confirmed cases and 139 confirmed deaths across 29 health zones, corresponding to a crude observed CFR of 20.2% (95% CI 17.2–23.4%). Ituri province accounted for 646 cases (93.8%) and had a CFR of 17.6% (95% CI 14.8–20.8%). By 10 June 2026, Uganda had recorded 19 confirmed cases and two confirmed deaths. In the DRC, apparent doubling times were similar during 21–27 May and 4–10 June 2026 (10.5 and 10.3 days, respectively), with a shorter estimate of 7.4 days during 28 May–3 June 2026. Among eligible Ituri health zones, CFRs ranged from 3.0% in Nyankunde to 38.2% in Mongbwalu. Contact follow-up reached 71.8% on 10 June. Ituri test positivity ranged from 26.6% to 68.5% across four reported time points. Official aggregate data from DRC and Uganda during the early 2026 BVD outbreak showed continued accumulation of confirmed cases, geographic expansion, crude outcome heterogeneity and operational constraints in contact follow-up, case management and laboratory turnaround. Phase-specific growth estimates were sensitive to surveillance-series representation and should be interpreted primarily as descriptive measures of reported case accumulation, not direct transmission estimates. Transparent documentation of revisions, unresolved outcomes and subnational data gaps is essential for credible real-time outbreak analysis.