Renewed Confirmed-Case Reporting in Health Zones After 21 Days Without a Newly Attributed Case During the 2026 Bundibugyo Virus Disease Outbreak in the Democratic Republic of the Congo: Implications for Health-Zone Surveillance and Response

<jats:p>Background: During the geographically dispersed 2026 Bundibugyo virus disease (BVD) outbreak in the Democratic Republic of the Congo (DRC), 21 days without a newly attributed confirmed case may inform health-zone surveillance and resource-allocation decisions while transmission continues elsewhere. However, the frequency and timing of renewed confirmed-case reporting after this interval have not been quantified. Methods: A retrospective cohort study was conducted among health zones affected by the 2026 BVD outbreak in the DRC. The primary data source was the national situation-report series, triangulated with World Health Organization weekly reports. Health zones that completed 21 days without a newly attributed confirmed case by 2 August 2026 entered the cohort. The observed recurrence proportion, incidence per 100 zone-days and Kaplan-Meier time to recurrence were estimated. Spatial adjacency to zones with reported confirmed-case activity during the 21 days before each focal health zone completed the interval was described. Results: Seventeen of 51 affected health zones completed the 21-day interval. Ten subsequently reported recurrence and seven were right-censored, giving an observed recurrence proportion of 58.8% (95% exact confidence interval 32.9% to 81.6%). The incidence was 2.83 recurrences per 100 zone-days (95% confidence interval 1.36 to 5.21). The estimated cumulative probability of recurrence was 20.0% by day 7, 41.3% by day 14, 48.7% by day 21 and 56.0% by day 28; median recurrence-free time was 22 days. All recurrent health zones and five of seven censored zones shared a boundary with a zone that had reported confirmed-case activity during the 21 days before threshold crossing. Aggregate reports could not distinguish persistent undetected transmission, reintroduction, delayed confirmation or retrospective attribution. Conclusions: Completion of 21 days without a newly attributed confirmed case was frequently followed by reported recurrence. The interval should not be used as a stand-alone trigger for substantial surveillance de-escalation. Decisions should also consider subsequent observation time, nearby case activity, surveillance performance and unresolved laboratory or reporting constraints.</jats:p>