In the trial, the vaccine had been found to be 100% effective. But the number of people enrolled was limited, and no vaccine works every single time. Still, the results were strong enough to convince Merck to push forward with the vaccine.

It did so with support from BARDA, which began funding rVSV-ZEBOV during the West African outbreak. The agency’s director, Rick Bright, estimated that it has spent about $175 million supporting production of vaccine and validation of Merck’s production facility for the vaccine in Germany.

When Ebola broke out in Equateur province in the Democratic Republic of the Congo in the spring of 2018, the country agreed to use the vaccine under a “compassionate use” protocol — similar to the protocol used in a clinical trial when there is no approved therapy. Vaccination began again in the country in the current outbreak, this time eight days after it was declared. Since then more than 260,000 people have been vaccinated.