Scientists are testing an Ebola vaccine against a virus it was never designed to prevent, as the Democratic Republic of Congo races to contain an outbreak that has now crossed 8,000 confirmed cases.
The experiment is testing whether Ervebo, a vaccine licensed against the Zaire species of Ebola virus, can also protect people exposed to the Bundibugyo virus the pathogen driving the current outbreak.
“Although ERVEBO was not originally developed to protect against Bundibugyo, preliminary data suggest that it may offer some degree of protection against the virus,” said Dr Guyguy Manangama, an epidemiologist and deputy director of operations at Médecins Sans Frontières, MSF.
“In a race against time such as the one the DRC is currently facing, we must use every tool available to us.”
MSF and its epidemiological research arm, Epicentre, launched the BRAVO vaccination study in Bunia, Ituri province, in collaboration with the DRC Ministry of Health, Africa CDC, the Congolese National Institute for Biomedical Research and other scientific partners.
The study will follow 20,000 frontline workers in Ituri and North Kivu, two areas where health workers and other responders face repeated exposure to the virus.
The research will run for nine to 12 months, with vaccination taking place over three months and participants followed for at least another six months.
The aim is not just to vaccinate workers but to generate evidence on whether Ervebo actually protects against Bundibugyo in real-world outbreak conditions.
WHO has made the scientific uncertainty explicit.
It says it is not yet known whether Ervebo protects humans against Bundibugyo virus. Laboratory and animal findings suggest possible cross-protection, but the evidence is not sufficient to establish whether the vaccine provides clinically meaningful protection in people.
WHO therefore recommends that Ervebo be used against Bundibugyo only within research protocols, while proven infection-prevention measures remain in place.
Manangama said even limited protection could make a difference.
“Even partial protection could help reduce severe illness and deaths,” he said. “However, these potential benefits must be demonstrated scientifically. That is precisely the purpose of BRAVO.”
The distinction between the two viruses is central to the study.
Ervebo was developed and licensed against Zaire ebolavirus, the species responsible for many of the major Ebola outbreaks recorded in Africa. The current DRC outbreak is caused by Bundibugyo ebolavirus, a different Ebola virus species.
That means protection against one cannot simply be assumed to work against the other.
WHO’s emergency guidance issued in August said some evidence points to possible cross-protection, but concluded that the available evidence remains insufficient to determine whether Ervebo protects humans against Bundibugyo.
The outbreak has given scientists little room to wait.
The outbreak has spread across 63 health zones in seven provinces. WHO said Ituri remains the most affected province, while North Kivu continues to report substantial transmission.
Two newly affected health zones, Dungu in Haut-Uélé, which borders South Sudan, and Bulu in Sud Ubangi, have also widened the geographical reach of the outbreak and increased concerns about cross-border transmission.
The vaccine study is therefore running alongside the wider response rather than replacing it.
MSF says vaccinated frontline workers must continue using protective equipment, following safe-care procedures and reporting symptoms quickly.
“It is important to stress that this vaccine will not end the current outbreak,” Manangama said. “However, it may offer additional protection to those most exposed, and help strengthen the response alongside other essential interventions.”
Several vaccines specifically designed to target the Bundibugyo virus are also under development.
