Summary
Background
On May 15, 2026, WHO declared a Bundibugyo virus (BDBV) outbreak in Ituri Province, DR Congo with an estimated index case on April 1, 2026 (6-week pre-declaration interval). By May 24, DR Congo reported 906 suspected cases (105 confirmed and ten confirmed deaths) across three provinces. Uganda reported seven confirmed cases (three imported; four locally acquired including three health-care workers; case-fatality ratio 14%). WHO declared a Public Health Emergency of International Concern on May 17, 2026; Africa Centres for Disease Control and Prevention declared a Public Health Emergency of Continental Security on May 18, 2026. The study aimed to establish a short-term trajectory of the BDBV outbreak and probability of cross-border spillover into countries with elevated risk of importation to guide preparedness priorities.
Methods
We calibrated a stochastic SEIRD (susceptible, exposed, infectious, recovered, and dead) ensemble model to the laboratory-confirmed case series, anchoring on 598 cumulative confirmed cases on June 8, 2026 (day 68) using simulation filtering (calibration window ±30%; reporting fraction 1·0 for laboratory-confirmed cases). The case-fatality ratio was drawn from a previous value centred on the observed confirmed-case ratio of approximately 19% (115 of 598). A linked daily-hazard spillover model estimated importation probability for Uganda, South Sudan, Rwanda, and Burundi over a 12-week horizon. The early suspected-case series, which peaked at 1077 on May 26, 2026, before being substantially revised downward by laboratory reclassification, is reported for context but was not used for calibration.
Findings
Laboratory-confirmed DR Congo cases rose from 33 on May 18, 2026, to 598 by June 8, 2026. Calibrated to the confirmed-case anchor (598 on June 8, 2026; central basic reproduction number [R0]=1·71), the confirmed-case trajectory is most consistent with the central scenario. Under the central scenario the ensemble projected a median of 990 cumulative confirmed cases by week 12 (June 24, 2026; 90% prediction interval [PI] 709–1293) and 174 deaths; the low scenario projected 870 confirmed cases (90% PI 641–1133) and 160 deaths. The early suspected-case count (peak 1077 on 26 May 2026) was substantially revised by laboratory reclassification and is reported for context only. Cross-border spillover remained material: Uganda 94·2% importation probability (19 confirmed cases as of June 4, 2026, including five health-care worker infections and two deaths); South Sudan 69·3%; Rwanda 8·6%; and Burundi 2·0%. As of June 22, 2026, DR Congo has 1048 confirmed cases and 267 confirmed deaths and Uganda has 20 confirmed cases, two confirmed deaths, and one probable death. These numbers are changing daily and are likely to align with what is predicted in the central scenario.
Interpretation
From the most recent laboratory-confirmed data, the outbreak is closer to what is predicted by the central scenario, even with the intensified response within DR Congo. However, uncertainty remains around reported case numbers due to low rate of contact tracing. Sustained control nonetheless remains the primary determinant of regional risk: importation into Uganda is already established, and South Sudan must continue to reinforce infection prevention and control, rapid response capacity, and cross-border surveillance under International Health Regulations 2005. These projections should be interpreted as exploratory preparedness-oriented estimates derived from a stochastic scenario-based modelling framework, rather than predictions generated from formally fitted epidemiological models using comprehensive parameter estimation and identifiability analyses.
Funding
None.
Translations
For the French and Swahili translations of the abstract see Supplementary Materials section.
Source:
Link: https://www.thelancet.com/journals/laninf/article/PIIS1473-3099(26)00320-8/fulltext
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