The latest Ebola outbreak in the Democratic Republic of the Congo, confirmed only at the end of last week, is now the fourth largest recorded. The virus is spreading in a conflict zone, complicating containment efforts. Currently, there is no vaccine targeting the specific virus species involved, Bundibugyo.
However, there is limited scientific evidence suggesting that Merck’s Ervebo vaccine, originally designed for the Zaire ebolavirus, may offer some protection against Bundibugyo.
The World Health Organization and virus experts are considering whether Ervebo could be effective in managing this outbreak. A meeting scheduled for Tuesday will bring together WHO advisors to discuss whether Ervebo should be tested in this context, according to Vasee Moorthy, acting lead of WHO’s R&D Blueprint group.
The use of the vaccine will ultimately depend on requests from the affected countries.
“Decisions on next steps will be made by the DRC and Uganda, with WHO’s support,” Moorthy stated, noting that ethical and other factors, such as participant willingness for clinical trials, must be considered.
The WHO has declared this rapidly expanding outbreak a public health emergency of international concern as of Sunday. By Monday, the African Centres for Disease Control and Prevention reported 395 suspected cases and 106 deaths in the DRC. Uganda has reported two cases among travelers from the DRC, with one fatality.
Evidence indicating Ervebo’s potential cross-protection against Bundibugyo comes from a small primate study. Opinions differ on whether these findings could translate into human protection and if testing the vaccine is justified.
Virologist Darryl Falzarano, lead author of a 2011 study published in the Journal of Infectious Diseases, remarked, “It’s a damned if you do, damned if you don’t situation,” indicating the challenges of proceeding with limited data.
In the study, three out of four vaccinated macaques survived a lethal dose of the virus, compared to one out of three unvaccinated animals, though all vaccinated animals showed symptoms. These results pose interpretation challenges, as noted by Ebola expert Tom Geisbert, who suggested the protective effect might be around 50% rather than 75% if some animals survived without vaccination.
Geisbert indicated that a larger study might offer more confidence in the vaccine’s potential benefits for humans. Despite this, some scientists see enough evidence to warrant further investigation of Ervebo’s protective capabilities against Bundibugyo.
Armand Sprecher, an emergency physician with Doctors Without Borders, emphasized using available resources: “The non-human primate study is a reasonable basis for saying let’s use this vaccine, because we’ve got it, and we know that it’s at least safe.”
Merck, the manufacturer of Ervebo, has not confirmed its willingness to test the vaccine in this outbreak. A spokesperson stated in an email that its efficacy against other strains, like Bundibugyo, is unknown due to limited data.
Concerns exist about testing the vaccine against Bundibugyo, with some scientists fearing unintended consequences. Although in vitro evidence suggests the possibility of antibody-induced enhancement, it has not been observed in animal studies.
Falzarano is more concerned about the vaccine potentially distracting the immune system from battling Bundibugyo if encountered. He would consider using Ervebo in a well-organized clinical trial, despite the challenges, but otherwise deems it “too risky.”
The decision remains complex due to limited data.
“To me, this isn’t a slam dunk,” Geisbert concluded.
