WHO Urges Phase III Trial of Ervebo Vaccine Against Bundibugyo Amid Major Ebola Outbreak

The World Health Organization's Technical Advisory Group on Candidate Vaccine Prioritisation (TAG-CVP) has recommended prioritising the Zaire ebolavirus vaccine Ervebo for evaluation in a Phase III trial against the circulating Bundibugyo strain driving the current outbreak.

TAG-CVP Recommendation

At a meeting on 31 July, the TAG-CVP concluded — with one dissenting view — that Ervebo should be considered for inclusion in a Phase III study targeting Bundibugyo. Panel members judged that the vaccine's extensive safety, reactogenicity and immunogenicity data, together with population-level safety experience, support progression directly to Phase III without a new Phase II trial.

Evidence and Limitations

The recommendation was informed by several preclinical and early research studies. The panel noted limitations in the evidence base, including that some reports were not peer-reviewed and that some experiments used research-grade rVSV-EBOV material rather than the licensed Ervebo product. Across four studies reviewed, Ervebo induced low levels of cross-reacting binding antibodies against Bundibugyo, indicating limited but detectable cross-recognition.

Safety, Supply and Trial Design Considerations

All members agreed there were no new safety concerns that would preclude using Ervebo in a clinical research setting. The vaccine is licensed by multiple regulators and has been administered to hundreds of thousands of people in Africa. WHO also maintains a stockpile of Ervebo, which could simplify rapid deployment for a ring-vaccination Phase III trial.

However, the TAG-CVP cautioned that expectations about likely efficacy should be tempered: Ervebo may offer limited protection against symptomatic infection and transmission of Bundibugyo, while it might still reduce the risk of death in vaccinated individuals. The panel emphasised that pursuing Ervebo now should not prevent later evaluation of a Bundibugyo-specific vaccine candidate, if one becomes available and is recommended by TAG-CVP.

Parallel Research Efforts

In early August, Moderna launched a first-in-human Phase I trial (NCT07737717) of its mRNA candidate, mRNA-1469, targeting the Bundibugyo variant. The Health Canada–approved study will run at three Canadian sites, enroll about 80 healthy adults, and assess safety, tolerability and immunogenicity.

Separately, WHO has initiated the PARTNERS therapeutic trial to evaluate antiviral and antibody-based treatments for infected patients. The trial will test Gilead Sciences' remdesivir (Velkury) and Mapp Biopharmaceutical's investigational double-antibody therapy MBP134, alone or in combination, with initial activities beginning in the Democratic Republic of Congo (DRC).

Outbreak Context

Bundibugyo virus is one of several viruses that cause Ebola disease and is responsible for the current DRC outbreak. While vaccines exist for Zaire ebolavirus, none are licensed specifically for Bundibugyo. The outbreak has been declared a Public Health Emergency of International Concern (PHEIC) by WHO and a Public Health Emergency of Continental Significance (PHECS) by the Africa Centres for Disease Control and Prevention (Africa CDC). It has resulted in more than 3,000 confirmed cases and over 1,400 deaths, making it one of the largest filovirus outbreaks on record.

Next steps: Implementation of a Phase III ring-vaccination study would require protocol development, regulatory approvals, community engagement and logistics planning to deploy Ervebo from existing stockpiles. Continued monitoring of Bundibugyo-specific vaccine candidates and therapeutic trial results will also inform the outbreak response.