“We found that’s probably the one that comes up the least,” Geisbert says. “We guessed wrong.”
Concerned about this gap in knowledge, in 2011 he decided to modify the vaccine, which led to research on the crab-eating macaque. The same study also ultimately tested a mix of existing Ebola vaccines on the Bundibugyo strain, but they did not provide 100% protection.
Geisbert says that if the 2012 outbreak had occurred after the vast outbreak in Zaire, it is possible that pharmaceutical companies would have been more willing to commercialize a vaccine to protect against the Bundibugyo strain.
However, with the current epidemic rivaling that of 2013-2016 in scale and scope, efforts to catch up will be challenging. Geisbert suspects that the WHO’s experience with Ervebo is part of the reason they favor its vaccine candidate, which is essentially “Bundibugyo Ervebo,” he says.
WHO also noted the success of a similar rVSV-based vaccine against the Sudanese strain of Ebola in a ring trial conducted in 2025.
The suitability of the Bundibugyo rVSV candidate for ring vaccination was confirmed in a 2023 study that showed that most monkeys were protected against the virus even after exposure if vaccinated. This is crucial for ring vaccination to work. Although researchers vaccinated the monkeys unrealistically quickly 20 minutes after exposure, the proof of concept sets this vaccine apart from those being developed by Moderna and the University of Oxford candidates.
“Not much has changed since the 2023 study because we didn’t really expect this burden and also because it has also historically been associated with lower mortality,” said Courtney Woolsey, the paper’s lead author (Geisbert was a co-author) and an assistant professor at the University of Texas School of Medicine.
“Nobody is really making money on these vaccines,” he adds, “so there are also barriers to financing the development of these vaccines that people probably won’t make money on.”
The nonprofit Coalition for Epidemic Readness Innovations has offered funding of up to $3.2 million to prepare and begin testing the material needed to produce Gesbert’s vaccine, which would be the first step toward human trials.
“Extensive safety data and prior regulatory experience” with rVSV-based vaccines used to combat the Zaire strain “could accelerate approval pathways if they are shown to be effective,” Rachael Bonawitz, filovirus disease program manager at CEPI, tells WIRED via email, adding that developers will also be able to build on existing manufacturing processes.
“Even if it doesn’t get used in this epidemic, hopefully there will be clinical material available that can be used in humans in the next epidemic,” Geisbert says, “because it will likely re-emerge.”
Even if it looks promising, there is still a risk that his vaccine won’t work. Scientists were unable to obtain a sample of live Bundibugyo virus for testing due to depleted resources in the DRC and the logistical and bureaucratic complexities associated with obtaining and transporting chilled blood back to the US. Although scientists believe the current strain is about 98 percent similar to the strain that caused previous outbreaks, that unknown 2 percent creates a risk that the vaccine will not be as effective as against the previous strain.
“When you look at the sequences, there isn’t enough variation to predict that there will be a problem, but nothing is foolproof,” Geisbert says.
The International AIDS Vaccine Initiative in Novel York will prepare the vaccine candidate for production. The nonprofit biomedical research organization focuses on developing vaccines for global diseases in areas where there are few financial incentives for development.
“The baton has been passed and I’m just sitting back and hoping it works, whether it’s the vaccine or someone else’s vaccine,” Geisbert says.