“We thought that was probably the least likely to happen,” Geisbert says. “We thought wrong.”
Concerned by the knowledge gap, in 2011 he decided to modify the vaccine, which led to a study of crab-eating macaques. In the same study, he finally tested a combination of ebola vaccines available in the Bundibugyo strain, but they did not provide 100 percent protection.
If the 2012 outbreak had happened after the big outbreak in Zaire, Geisbert says, it’s possible pharmaceutical companies would have been interested in commercializing a vaccine that protects against the Bundibugyo strain.
But with the current explosion rivaling that of 2013 to 2016 in terms of size and scope, catch-up efforts are accelerating. Geisbert suspects the WHO’s experience with Ervebo is one of the reasons they favor his vaccine candidate, which is essentially “Bundibugyo Ervebo,” he says.
WHO also noted the success of a similar rVSV-based vaccine targeting the Sudanese ebola strain in a ring vaccine trial in 2025.
The suitability of the Bundibugyo rVSV candidate for ring vaccination was supported by a 2023 study that showed most monkeys were protected from the virus even after exposure if they had been vaccinated. That is necessary for the ring vaccine to work. While the researchers vaccinated the monkeys with an unrealistically fast 20 minutes after exposure, the proof of concept sets it apart from Moderna and the University of Oxford’s developing candidates.
“There hasn’t been a lot of progress since that 2023 study, because we didn’t really expect to see that problem and also because it’s historically associated with low mortality as well,” said Courtney Woolsey, lead author on the paper (Geisbert was a co-author) and assistant professor at the University of Texas Medical Branch.
“Nobody is making money for these vaccines,” he adds, “so there are funding barriers as well to developing these vaccines where people likely won’t make money.”
The nonprofit Alliance for Epidemic Preparedness Innovations has provided up to $3.2 million in funding to develop and begin testing the materials needed to develop Gesbert’s vaccine, which would be the first step toward human trials.
“Extensive safety data and early regulatory experience” from the rVSV-based vaccines used against the Zaire strain “could help speed up the approval process if it proves successful,” Rachael Bonawitz, leader of the filovirus program at CEPI, tells WIRED via email, adding that manufacturers will also be able to continue existing manufacturing processes.
“Even if it’s not used in this outbreak, hopefully there will be clinical material that can be used in humans available for subsequent outbreaks,” Geisbert says, “because it will probably happen again.”
Even if it shows promise, there’s still a chance its vaccine won’t work. Scientists have not been able to obtain a live sample of the Bundibugyo virus for testing due to the limited resources in the DRC and the logistical and bureaucratic complexity of obtaining and transporting frozen blood to the United States. While scientists believe that the current strain is about 98 percent the same as the strain that caused the previous outbreaks, that unknown 2 percent presents the risk that the vaccine will not be as effective as it was against the previous strain.
“When you look at the sequence it’s not different enough that I would predict there would be a problem, but nothing out of the ordinary,” Geisbert says.
The International AIDS Vaccine Program in New York will prepare the vaccine candidate for production. The non-profit medical research organization focuses on developing vaccines for global diseases where there is little financial incentive for development.
“The stick is given out, and I just sit back and hope that it works, whether it’s the vaccine, whether it’s someone else’s vaccine,” Geisbert says.
