Existing Ebola vaccines might offer some protection against the rare Bundibugyo virus. 

The findings, published yesterday in the New England Journal of Medicine, offer hopeful news as Bundibugyo is fueling the fastest-growing Ebola outbreak on record, claiming more than 1,000 lives in the Democratic Republic of the Congo since February.

Bundibugyo belongs to a group of viruses that cause Ebola disease. A phase 1 trial to assess a vaccine that specifically targets Bundibugyo was launched earlier this month by the University of Oxford, as currently there are only licensed vaccines for the Zaire Ebolavirus, the most common of the six known Ebola species

To assess whether the Zaire vaccines could be used in the current outbreak, researchers took serum samples from 179 West African adults and children vaccinated with one of two licensed vaccines to see whether there was any antibody response against Bundibugyo. While not as strong as for Zaire, the Bundibugyo samples showed some cross-reactive immunity.

Specifically, three months after a single dose of Merck's Ervebo vaccine, the antibody response was about eight times lower for Bundibugyo compared with Zaire. At the same point, the two-dose Ebola vaccine regimen produced by a subsidiary of Johnson & Johnson—Zabdeno and Mvabea—elicited six times more antibodies for Zaire than for Bundibugyo.

The authors note that their results are consistent with those in studies involving nonhuman primates, and that the Merck vaccine, which is currently available through the global Ebola vaccine stockpile, "represents an immediately deployable candidate that could be evaluated during the ongoing outbreak."

The findings are not proof that these vaccines provide protection against Bundibugyo, but are still encouraging, said Boghuma K. Titanji, MD, DTM&H, PhD.

In a thread posted on the social media platform Bluesky, Titanji said that in the absence of a licensed vaccine, the data should prompt the World Health Organization (WHO) to revisit whether to deploy stockpiles of the Zaire Ebola vaccine. 

I do not think this outbreak affords us the luxury to wait much longer.

At this point, the Bundibugyo outbreak is the third-largest Ebola outbreak on record.

Titanji noted that rarely do public health officials and clinicians have "perfect evidence" when responding to an infectious disease outbreak. Decision makers must balance uncertainty against "the very real cost of waiting."

"I do not think this outbreak affords us the luxury to wait much longer," she said.