A vaccine for the Ebola strain driving the outbreak in the Democratic Republic of Congo and Uganda is likely months away from human trials, and there is no guarantee it would work, the World Health Organization said Wednesday.

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There are currently no approved vaccines for the Bundibugyo strain of the Ebola virus.

There are two potential vaccine candidates, but neither is ready to move into human testing, Dr. Vasee Moorthy, the lead for the WHO’s research and development blueprint, said during a news conference.

The more promising of the two could take six to nine months before enough doses are ready for trials, Moorthy said. The other may be available in two to three months but has not yet shown supporting results in animal studies.

“It will depend on the animal data as to whether that is considered a promising candidate vaccine for Bundibugyo,” he said.

The timeline has pushed health officials to consider other options, including Merck’s Ebola vaccine, called Ervebo.

Ervebo targets the Zaire strain, the most common and deadliest type of Ebola.

There is some evidence from animal studies suggesting Ervebo might offer some protection against the Bundibugyo virus — although Ebola experts say the existing data is limited and more research is needed.

“It is not a vaccine for Bundibugyo,” Moorthy said Wednesday, adding that a review on whether Ervebo could help mitigate the current outbreak is underway.

A 2011 study published in the Journal of Infectious Diseases found that an early version of Ervebo provided limited protection against the Bundibugyo virus in macaques. Three of four vaccinated animals survived Bundibugyo exposure, compared with 1 in 3 unvaccinated animals, though all vaccinated macaques developed symptoms.

Taking into account that some of the macaques may have survived without the vaccine, the protection from the vaccine might be lower, closer to 50%, said Tom Geisbert, an Ebola subject matter expert for the WHO and an author on the 2011 study.

“It’s a coin flip,” Geisbert said. “Unfortunately with these nonhuman primate studies … they’re just not large and you don’t have the statistical power.”

Until now, global health officials had found little need for a Bundibugyo vaccine, said Dr. Geeta Sood, a hospital epidemiologist at Johns Hopkins Bayview Medical Center.

Previous outbreaks of the Bundibugyo virus were small, Sood said, rare and relatively easy to contain.

The Bundibugyo virus has a mortality rate of around 25% to 40%, she said, lower than other types of Ebola, which average around 50% to 60%.

Without a vaccine, public health officials go “pretty much back to the basics of Ebola,” said Alan Gonzalez, deputy director of operations at Médecins Sans Frontières (known in English as Doctors Without Borders), “which is contact trace and identification, safe isolation of patients, being able to implement infection prevention and control in all the places where we work.”