The WHO has recommended a Phase III ring vaccination trial of the licensed Ervebo vaccine to test possible cross-protection against the Bundibugyo Ebola strain behind the DRC outbreak. Early animal data — including primate and ferret studies — suggest potential protection, and experts found no new safety concerns. The outbreak has reported 4,053 confirmed cases and 1,850 deaths; Gavi will provide doses from its 500,000-dose Ervebo stockpile for the trial.
WHO Calls for Phase III Ring Trial of Ervebo to Test Protection Against DRC's Bundibugyo Ebola Strain
Ervebo is the only licenced Ebola vaccine (Saidu BAH)
The World Health Organization has recommended launching a Phase III ring vaccination trial of the licensed Ervebo vaccine to assess whether it offers cross-protection against the Bundibugyo Ebola strain driving the ongoing outbreak in the Democratic Republic of Congo (DRC).
Background
Ervebo is approved and proven safe and effective against the Zaire Ebola virus. With more than 4,000 confirmed cases in the current DRC outbreak, WHO vaccine advisers revisited emerging data to determine whether Ervebo could help limit illness and death from the rarer Bundibugyo strain.
What Experts Reviewed
WHO's Technical Advisory Group on Candidate Vaccine Prioritisation (TAG-CVP) examined animal and early clinical data. Key findings included:
- In a non-human primate study, three of four vaccinated primates survived Bundibugyo infection versus one of four in the control group.
- An unpublished ferret study of a research-grade Ervebo showed 100% protection in vaccinated animals while all controls died within 10 days.
TAG-CVP concluded there are no new safety concerns for using Ervebo in a clinical study vaccinating contacts of confirmed cases, noting the vaccine's regulatory approvals and prior use in hundreds of thousands of people in Africa.
Managing Expectations
Experts cautioned that, if evaluated in a Phase III ring vaccination trial, Ervebo's ability to prevent symptomatic infection and transmission of Bundibugyo may be limited. However, it could still reduce the risk of severe disease and death. Final-stage trials typically enroll hundreds to thousands of participants and are designed to measure clinical efficacy.
Stockpile, Logistics and Next Steps
Gavi, the global vaccine alliance, has a stockpile of 500,000 Ervebo doses and has agreed to make doses available for the proposed trial; some shipments are already in the DRC. Approximately 55,000 frontline workers in Ituri and North Kivu have previously received Ervebo in preventive campaigns.
As of the latest update, the outbreak has reached 4,053 confirmed cases with 1,850 deaths. Around 87% of cases are concentrated in Ituri province and about 11% in North Kivu.
Outlook
While a Bundibugyo-specific vaccine remains the preferred long-term tool—two candidates are in early human trials and a third is under development—the recommended Ervebo ring trial could provide timely data on whether existing licensed vaccines can reduce severe outcomes in the current emergency.
Key risk: Ebola spreads through contact with bodily fluids and causes severe haemorrhagic fever; rapid containment and vaccination of contacts remain critical to limiting transmission and deaths.
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