The experimental Ebola vaccine being used to try to contain the outbreak in the Democratic Republic of the Congo is protective 97.5% of the time, according to new data released by the World Health Organization on Friday.

The data — the first to be released on how the vaccine is working in the outbreak — suggest a “very impressive” performance by the vaccine, said Michael Osterholm, director of the University of Minnesota’s Center for Infectious Diseases Research and Policy, who reviewed the report for STAT.

Of more than 90,000 people who were vaccinated, only 71 went on to develop Ebola. Fifty-six of those people developed symptoms fewer than 10 days after being vaccinated, suggesting the vaccine had not yet had time to fully protect them. It is believed it takes about 10 days for immune protection to develop after vaccination.

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Only 15 people developed Ebola more than 10 days after being vaccinated, the report said. Seven of them were health-care workers. Nearly 29,000 health-care and front-line workers were among the roughly 90,000 vaccinated between last August and March 25, the time period covered by the report.

None of the people who developed Ebola 10 days or more after being vaccinated died from their illness.

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“Notably, there were nine deaths among 56 cases with onset of symptoms 0 to 9 days after vaccination [who are assumed to be only partially protected by vaccination] and, no deaths among people where the illness onset occurred 10 days or more after they were vaccinated [who are assumed to be protected by vaccination],” the report said.

The data were posted on the WHO’s website following a meeting of a panel of experts asked to advise Director-General Tedros Adhanom Ghebreyesus on whether the outbreak in the DRC should be declared a public health emergency of international concern. The committee concluded that the epidemic, though increasingly serious, does not constitute a global threat at this time.

The outbreak, which was first recognized last August, is the second largest on record. As of Thursday there have been 1,206 cases and 764 deaths. Nearly 98,000 doses of vaccine have been administered over the past eight months.

The WHO has been under pressure to release data on the vaccine’s effectiveness. Osterholm said it was critical that the agency share what it is finding.

“I think it’s very important because it helps give the world a better sense of how effective this vaccine is. And that the real challenges are not about ‘Does the vaccine work?’ It’s whether the vaccine gets used,” he said.

“The problem is not because you have an inferior vaccine response. Frankly this is outstanding. The challenge is getting it into people.”

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Dr. Mike Ryan, executive director of WHO’s health emergencies program, agreed, saying the recent spike in cases in this outbreak was the result of the outbreak response team’s inability to vaccinate more people at the current hot spots of Katwa and Butembo, because many parts of those cities have been too dangerous for vaccination teams to operate in of late.

“We fell behind,” Ryan said in an interview. “And we’re paying the price for that now with increased transmission.”

The vaccine, which is being developed by Merck, is being used in what is known as a ring vaccination strategy. People who have had close and high-risk contact with known Ebola cases are offered vaccine. The contacts of those contacts are also offered a chance to be vaccinated. The aim is to throw up a wall of protection, cutting off the virus’s ability to spread.

People at high risk of exposure because of their jobs — health workers, ambulance drivers and people who man the safe burial teams — are also offered vaccine.

While the vaccine is still unlicensed, a clinical trial conducted in Guinea near the end of the West African Ebola outbreak showed the vaccine was highly efficacious. Based on that study, the WHO’s vaccine advisory committee has recommended that the vaccine be used as part of the arsenal of tools deployed to quell outbreaks.

The latest data reaffirm the findings of the trial in Guinea — though the report noted that this is an observational study, which is a limitation of this type of research.

The gold standard of trials involves randomly assigning some people to receive the vaccine and others to receive a placebo injection and comparing the infection rates in the two groups. But as this vaccine has been determined to be effective, it would be unethical to offer placebo injections rather than the vaccine.