Historic Phase III Trial of Ervebo Vaccine Launches Amid Rapidly Expanding Bundibugyo Ebola Outbreak in DRC

Historic Phase III Trial of Ervebo Vaccine Launches Amid Rapidly Expanding Bundibugyo Ebola Outbreak in DRC

In August 2026, the Democratic Republic of the Congo (DRC) and the World Health Organization (WHO) initiated a historic Phase III clinical trial of Merck’s Ervebo vaccine to evaluate its potential cross-protection against the rare Bundibugyo ebolavirus species. The trial begins as the DRC faces what is on track to become the largest Ebola outbreak on record.

As of August 1, 2026, the outbreak in the northeast of the country had reached 3,748 confirmed cases, including 1,657 deaths, making it the second-largest Ebola epidemic in history and the fastest-moving outbreak to date. By mid-August, the death toll had surged past 2,000. Currently, there is no approved vaccine or therapy specifically indicated for the Bundibugyo species of Ebola.

The Ervebo Cross-Protection Trial

Ervebo is a highly successful, live-attenuated vaccine developed and licensed specifically for the Zaire ebolavirus strain. Because of the urgent biosecurity crisis, global health authorities are taking an unprecedented approach to evaluate whether Ervebo can offer cross-protective immunity:

• Unanimous Recommendation: On July 31, 2026, the WHO's Technical Advisory Group on candidate vaccine prioritisation (TAG-CVP) unanimously recommended prioritizing Ervebo for inclusion in a Phase III clinical trial in the DRC.
• Scientific Rationale: The recommendation was supported by laboratory and animal studies (including serum analyses published in The New England Journal of Medicine in July 2026) showing that antibodies generated by Ervebo might offer cross-protection against the Bundibugyo strain.
• Accelerated Path: Given the extensive safety, reactogenicity, and immunogenicity data already available for Ervebo across large populations, the WHO advised that the vaccine could bypass Phase II testing and enter directly into a Phase III field trial.¹
• Dose Arrival: On August 20, 2026, the WHO and Africa CDC welcomed the arrival of 70,000 doses of Ervebo in the DRC, enabling the immediate rollout of the trial to frontline healthcare workers and high-risk contacts.

Fast-Tracked Bundibugyo Candidates

While testing Ervebo for cross-protection represents an immediate containment strategy, researchers are also advancing clinical trials for experimental vaccines designed specifically for the Bundibugyo strain. Supported by the Coalition for Epidemic Preparedness Innovations (CEPI), Moderna initiated a Phase I trial of its experimental mRNA-based Bundibugyo Ebola vaccine in August 2026, alongside other candidates from Oxford (ChAdOx1) and IAVI (rVSV).