Emergency teams responding to the ongoing Ebola outbreak in Congo began on May 21 inoculating those most at risk of contracting the virus: health workers and people who have come into contact with Ebola victims. It’s the first real-world test for an experimental vaccine, rVSV-ZEBOV. In field trials in Guinea and Sierra Leone in 2015, this vaccine effectively protected people from Zaire ebolavirus — the same type of Ebola now circulating in Congo.

In the latest outbreak, 51 people have developed cases of hemorrhagic fever consistent with Ebola, and 27 have died.  The outbreak is centered in the rural Bikoro region but nearly a handful of cases have been reported in the city of Mbandaka.

Using a “ring vaccination” strategy, health care workers are offering shots not just to the people who’ve had contact with Ebola victims, but also to a second ring of people who’ve had contact with the first group. In that way, the World Health Organization and its partners hope to disrupt the chain of transmission.

Merck, the company that makes the vaccine, has donated 8,640 doses to the emergency response effort. That’s more than enough for 50 rings of 150 people. Another 8,000 doses are expected to become available soon, according to the WHO.

Health officials are optimistic. “Today marks a turning point in how we deal with Ebola,” WHO Deputy Director-General for Emergency Preparedness and Response Peter Salama said on Twitter.