What does a health-zone case-fatality ratio measure during an active outbreak? Reported mortality, mapped health-facility context, and case-death reporting heterogeneity in the 2026 Bundibugyo virus disease epidemic in DR Congo

Abstract Background. During an active outbreak, deaths divided by confirmed cases can be mistaken for biological severity or quality of care even though outcomes remain unresolved and reporting and ascertainment differ across places. We examined what health-zone case-fatality ratios measured during the 2026 Bundibugyo virus disease epidemic in the Democratic Republic of the Congo. Methods and principal findings. We analysed national and health-zone cumulative confirmed cases and deaths through 20 July 2026. We calculated reported crude case-fatality ratios, examined case-death reporting synchronisation, and fitted beta-binomial partial-pooling models with four-chain Markov chain Monte Carlo. Exploratory models added mapped clinical-facility availability or distance to the nearest mapped hospital. Reporting-date delay, under-ascertainment, and mortality forecasting were evaluated as diagnostics, scenarios, or developmental analyses rather than patient-level fatality estimation. The national ratio reached 40.4% (999/2,473). Among 14 zones with at least 20 cases, crude ratios ranged from 28.3% to 68.5%. In the primary model, North Kivu had higher posterior odds than Ituri, but with substantial uncertainty (OR 1.80, 95% credible interval 0.92-3.37); epidemic maturity was positively associated (1.49, 1.04-2.19). In exploratory models, greater mapped clinical-facility availability was associated with lower reported fatality (0.67, 0.48-0.96), while greater distance to a mapped hospital was associated with higher reported fatality (1.46, 1.06-2.00). Same-day case-death co-reporting was common, and materially different reporting-delay assumptions fitted similarly. Interpretation. Health-zone ratios revealed meaningful surveillance heterogeneity but did not identify biological fatality risk or causal effects of facilities, access, care, or conflict. Epidemic maturity, selective ascertainment, referral, and administrative reporting plausibly shaped the numerator and denominator. These ratios should guide investigation rather than rank health-zone performance; linked patient records are required for clinical fatality and competing-risks analyses.