Key Insights
- The Democratic Republic of the Congo is receiving 70,000 doses of the Ebola vaccine, Ervebo, in a Phase 3 clinical trial to determine the effectiveness of the vaccine against a species of Ebola that it wasn’t specifically designed to target.
- New evidence has emerged supporting the cross-reactivity of Ervebo against the Bundibugyo virus, which is responsible for the current outbreak.
- It’s not yet clear if Ervebo will be effective in the context of the current outbreak.
The World Health Organization (WHO) has announced that 70,000 doses of Merck & Co.’s Ebola vaccine, Ervebo, have made it to the Democratic Republic of the Congo (DRC) and will be tested in a Phase 3 trial. Ervebo was approved by US and European regulatory agencies to vaccinate against Orthoebolavirus zairense, the most common species of Orthoebolavirus; it has not yet been evaluated for effectiveness in humans against the Bundibugyo virus, Orthoebolavirus bundibugyoense, the related but rarer species now circulating in the DRC.
The WHO says 20,000 doses have been allocated for the Phase 3 clinical trial, while the other 50,000 doses will be given to frontline health workers.
The vaccines were delivered following a request last week from the government of the DRC to the International Coordinating Group on Vaccine Provision, which manages the stockpile of Ebola virus vaccines. “They’re the country that’s affected by the outbreak, and so they’re trying to do what they can and take care of their people,” says Krutika Kuppalli, an infectious disease physician who ran an Ebola clinic in Sierra Leone during the 2014–16 Ebola virus outbreak.
Kuppalli was the lead author of a comment published in the Lancet Infectious Diseases in July calling for Ervebo to be reconsidered for use in the current DRC outbreak (2026, DOI: 10.1016/S1473-3099(26)00382-8). The comment cites data from the DRC Ministry of Health DHIS2 tracker showing that nine individuals infected with Bundibugyo virus had previously been vaccinated with Ervebo, and all survived.
Ervebo presents the immune system with a modified Ebola virus glycoprotein that is about 35% different from the Bundibugyo virus glycoprotein, according to Kristian Andersen, an immunologist at Scripps Research, who spoke to C&EN in May, soon after the start of the outbreak. That difference could mean Ervebo won’t work as well against Bundibugyo virus.
In 2011, a paper suggested that Ervebo offered a modest protective effect against Bundibugyo virus in a small sample of nonhuman primates (J. Infect. Dis., DOI: 10.1093/infdis/jir350). Since the start of the current outbreak, multiple new lines of evidence have emerged supporting the potential efficacy of Ervebo.
A WHO Technical Advisory Group meeting on July 31 cited three studies, published since the beginning of the outbreak, showing some ability for Ervebo to stimulate a response to the Bundibugyo virus glycoprotein in laboratory testing, albeit less of a response than to the Ebola virus glycoprotein (medRxiv 2026, DOI: 10.64898/2026.05.07.26352689 and 10.64898/2026.05.27.26354223; Lancet Inf. Dis. 2026, DOI: 10.1016/S1473-3099(26)00407-X ). Those studies, along with some unpublished data, were cited as reasons to evaluate Ervebo’s effectiveness against Bundibugyo virus in the current outbreak.
An additional study, in ferrets (Emerg. Inf. Dis. 2026, DOI: 10.3201/eid3208.260948), and a study on serum samples from previously vaccinated individuals (medRxiv 2026, DOI: 10.64898/2026.06.22.26356273) also provide some evidence for Ervebo’s effectiveness against both species.
Several Bundibugyo-specific vaccines are also in development, but none are ready for deployment in the current outbreak. In July, University of Oxford researchers began the first human trial of a Bundibugyo-specific vaccine, ChAdOx1 BDBV. And in August, the first humans received doses of Moderna’s messenger RNA–based Bundibugyo vaccine, mRNA-1469.
Kuppalli says that it will likely take some time before those vaccines are ready for deployment but that they might still make it to trial in time to help with the current outbreak. “Given how bad the outbreak is, I suspect that those vaccines will be available at some point during the current outbreak,” she says.
But the multiple vaccines add complexity to data analysis and participant education. Kuppalli says it will be important to have conversations with those receiving Ervebo so they “understand that they’re getting a vaccine that shows efficacy against Ebola but not particularly the strain that is circulating right now, and therefore it may not be as protective.”
The WHO said it could not respond to C&EN’s questions about the vaccine trial before this story’s deadline.
