An experimental vaccine against the Ebola virus is 97.5 percent effective at preventing the disease, protecting well over 90,000 people in the massive, ongoing outbreak in the Democratic Republic of the Congo, according to preliminary data.

The outbreak has flared since last August, involving 1,264 cases (1,198 confirmed; 66 probable) and 814 deaths (748 confirmed, 66 probable), making it the second-largest Ebola outbreak recorded. So far the outbreak has stayed within the DRC’s North Kivu and Ituri provinces, which sit on the eastern side of the country, bordering South Sudan, Uganda, and Rwanda. But, response efforts have been severely hampered by community distrust of public health campaigns. One result of this distrust has been several attacks by militants on medical facilities, injuring medical staff and, in one case, killing a police officer. Some public health experts fear the outbreak will continue to spread without new strategies and more aid, possibly across nearby borders.

Still, the outbreak could have been far worse if it had not been for an experimental vaccine. The rVSV-ZEBOV-GP Ebola vaccine, made by Merck & Co, contains a live attenuated virus harmless to humans that researchers genetically engineered to carry an Ebola glycoprotein. Ebola usually uses this protein to interact with human cells, but in the vaccine, it triggers the human immune system to generate powerful antibodies to attack the virus. Early tests of the vaccine seemed to confirm this, suggesting it is safe and effective. And a World Health Organization Strategic Advisory Group of Experts (SAGE) has given responders the greenlight to use the vaccine during outbreaks, based on an Expanded Access/Compassionate Use protocol.

As in the last outbreak, health responders are currently unleashing it in a ring vaccination strategy, which aims to immunize those in contact with known Ebola cases and contacts of those contacts—creating social rings of immunization to protect the most vulnerable and prevent the spread of disease. This was the same vaccination strategy used in the eradication of smallpox, the only human disease to ever be fully wiped out. Though Ebola spreads from animals—thus couldn’t be eradicated from a human vaccination campaign alone—the strategy is considered the most effective at quickly and efficiently extinguishing any flare ups and outbreaks.

That appears to be the case in the current outbreak, according to the preliminary data released by WHO and DRC researchers (PDF). Between August 1, 2018 to March 25, health responders mapped out 679 rings around 776 of the 951 confirmed and probable cases during that time period. They vaccinated nearly 94,000 people, including nearly 29,000 healthcare and front-line responders in that time period (more have been vaccinated since then).

Of those vaccinated, only 71 people fell ill with Ebola. Most of those cases (54 of 71) were in high-risk contacts—those thought to be most likely to come down with the virus based on exposure. Moreover, most of them (56 of 71) occurred within 10 days of being immunized, before the vaccine is thought to induce full protection. Only 15 of the nearly 94,000 people vaccinated became ill after that 10-day period, and all of them survived the infection, which usually has a fatality rate of around 50 percent.

Looking at the rings overall, researchers found that Ebola was able to move through only about 9 percent of the vaccinated rings (60/679 rings), and only about two percent of those rings included cases after the 10-day period. Last, only two vaccinated people out of the 68,279 vaccinated people listed as “contacts-of-contacts” fell ill with the virus. This suggests that the vaccine and the vaccination strategy were highly effective at preventing the spread of the disease.

In all, based on calculations comparing Ebola’s spread through vaccinated and unvaccinated rings in the outbreak, researchers estimated the virus’ attack rate was 0.017 percent among the vaccinated and 0.656 percent among unvaccinated. That yields an estimated efficacy rate of 97.5 percent.

In a meeting late last week, WHO experts assessed the outbreak overall and determined that it did not constitute a “Public Health Emergency of International Concern,” though they still expressed “deep concern” over the virus’ continued spread. They called for continued vaccination and a redoubling of efforts to work with communities to stamp out the disease.