Cross-border travel patterns affect magnitude estimates for the Ebola Bundibugyo epidemic

A geographic spread approach is often used to estimate the true burden of infectious disease cases in a source population by leveraging statistical signals generated by cases exported and reported elsewhere. Here we estimated an upper bound on the likely burden of Bundibugyo virus disease cases in the Democratic Republic of Congo early in the unfolding 2026 epidemic based on the number of cases imported and confirmed in Uganda. We expanded a geographic spread approach to account for its specific epidemiological contexts: most cross-border movements between the Democratic Republic of Congo and Uganda are short-duration trips, and, at the time of this analysis, all cases imported and confirmed in Uganda were individuals who travelled specifically to seek healthcare. Incorporating these factors substantially affected burden estimates, highlighting both the potential severity of the epidemic in its early phase and the critical need to consider local epidemiological contexts when applying geographic spread approaches to reduce and be aware of potential biases in estimates for early epidemic responses.