The fever was first symptom to grip crab-eating macaques in their highly confined laboratory on an island off the coast of Texas after they were infected with the recently discovered Bundibugyo strain of Ebola. Then came weight loss, rectal and nasal bleeding, while scientists in spacesuits drew blood to see how the monkeys’ immune systems struggled to fight off the aggressive virus.
But the three monkeys who had received a newly developed vaccine to protect against the little-studied strain showed no symptoms of the disease, which ultimately killed two-thirds of their unvaccinated companions.
It was 2011, and virologist Thomas Geisbert’s work to develop the vaccine was complete. If the vaccine had protected the primates from Bundibugyo strain of Ebolait was very likely that he was protecting humans. Yet with a the epidemic is now raging In the Democratic Republic of Congo and Uganda, Geisbert’s promising vaccine has not been rolled out at all – or even put into human trials – due to lack of funding or interest.
And it could take months to test its safety and effectiveness, even as the Bundibugyo virus causes widespread suffering. “We have the rVSV Bundibugyo vaccine on the shelves,” says Geisbert, professor of immunology at the University of Texas Medical Branch at Galveston. Recombinant vesicular stomatitis virus “rVSV” vaccines use a harmless version of this virus to provide the genetic instructions the body needs to fight the disease.
Hundreds of people were infected in the current outbreak in central and eastern Africa, and around 200 people have died. Public health officials have been scrambling to develop a vaccine, with the World Health Organization identifying Geisbert’s as the most promising candidate.
Geisbert’s work began in the early 2000s as an advocacy project focused on other strains of Ebola. Following the September 11 attacks and fears that terrorists might use the Ebola virus and other similar pathogens as biological weapons (a topic the Soviet Union had investigated during the Cold War), the U.S. military funded the development of a vaccine against the virus.
His first big discovery in 2003 revealed that monkeys could be protected against the Ebola virus with a single injection of the vaccine he developed. But when Geisbert first published his findings a few years later, he found little commercial interest.
“There simply was no global market for an Ebola vaccine,” he says. “It’s not a source of income, no one really wanted to take it.”
This in part led Geisbert to examine whether this vaccine could protect monkeys against different strains of the disease, making it cheaper and easier to develop and mass produce. He tested a mixture of vaccines against three of the four Ebola viruses known to harm humans successfully and published the results in 2009.
Interest in taking them beyond the laboratory has reached critical mass over the 2013–2016 Ebola outbreakwhen the Zaire strain, the most common, infected 28,600 people and killed 11,300 in West Africa. The rapid spread of the virus and its high mortality rate have sparked a race to develop a vaccine. This included one developed by pharmaceutical giant Merck in part through Geisbert’s work. Called Ervebo, it was deployed in a “ring” where contacts of infected people are vaccinated, thus creating a buffer zone limiting the spread of the virus.
The vaccine’s success earned Geisbert a spot among Time magazine’s “Ebola Fighters,” whom the publication dubbed its Person of the Year in 2014.
But Geisbert’s initial study omitted one strain of Ebola, Bundibugyo, because it has lower mortality rates and has caused only three outbreaks. This includes a 2012 outbreak that killed 30 people in about three months in the DRC, but was contained relatively quickly through contact tracing and isolation.
“We thought that was probably the one that was least likely to appear,” says Geisbert. “We guessed wrong.”
Concerned about this lack of knowledge, he decided in 2011 to modify a vaccine, which led to the study on crab-eating macaques. In the same study, he ultimately tested a mixture of existing Ebola vaccines on the Bundibugyo strain, but they did not provide 100% protection.
If the 2012 outbreak had occurred after Zaire’s major outbreak, Geisbert says, it’s possible that pharmaceutical companies would have been more inclined to market a vaccine that protects against the Bundibugyo strain.
But with the current outbreak rivaling that of 2013 to 2016 in scale and scope, catch-up efforts are shifting into high gear. Geisbert suspects that the WHO’s experience with Ervebo is one of the reasons it favors its vaccine candidate, which is essentially “Bundibugyo Ervebo,” he says.
The WHO also noted the success of a similar rVSV-based vaccine targeting the Sudanese strain of Ebola virus in a ring vaccination trial in 2025.
The suitability of the rVSV-based Bundibugyo candidate for ring vaccination was supported by a 2023 study showing that most monkeys were protected against the virus even after exposure if they had been vaccinated. This is crucial for ring vaccination to work. While the researchers vaccinated the monkeys within an unrealistically short time of 20 minutes after exposure, the proof of concept sets it apart from Moderna and candidates under development at the University of Oxford.
“There hasn’t really been a lot of development since that 2023 study, because we didn’t really expect to see this strain and also because historically it has also been associated with a lower mortality rate,” said Courtney Woolsey, lead author of the paper (Geisbert was a co-author) and an assistant professor at the University of Texas Medical Branch.
“No one is really making money from these vaccines,” she adds, “so there are also funding barriers to advancing these vaccines where people are unlikely to make money.”
The nonprofit Coalition for Epidemic Preparedness Innovations has offered funding of up to $3.2 million to prepare and begin testing equipment needed to make Gesbert’s vaccine, which would be the first step toward human trials.
The “extensive safety data and previous regulatory experience” of rVSV-based vaccines used to combat the Zaire strain “could help expedite approval procedures if they prove effective,” Rachael Bonawitz, head of the filovirus disease program at CEPI, told WIRED by email, adding that developers could also rely on existing manufacturing processes.
“Even if it’s not used in this outbreak, we hope there will be clinical material that can be used in humans for the next outbreak,” Geisbert says, “because it will likely reappear.”
Even though this seems promising, there is always a chance that his vaccine may not work. Scientists were unable to obtain a live sample of the Bundibugyo virus for testing due to limited resources in the DRC and the logistical and bureaucratic complexity of obtaining and transporting refrigerated blood to the United States. While scientists estimate that the current strain is about 98 percent similar to the strain that caused previous outbreaks, that unknown 2 percent poses a risk that the vaccine may not be as effective as it was against the previous strain.
“When you look at the footage, it’s not different enough for me to predict there would be a problem, but nothing is foolproof,” Geisbert says.
The International AIDS Vaccine Initiative in New York will prepare the vaccine candidate for production. The nonprofit biomedical research organization focuses on developing vaccines for global diseases for which there are few financial incentives for development.
“The baton has been passed, and I hope it works, whether it’s the vaccine or someone else’s,” Geisbert says.