Multiple health experts tested the vaccine in 2015 on almost 6,000 people in Guinea. After at least 10 days, not one of those people had developed symptoms of Ebola.
Researchers chose 10 days specifically because symptoms are known to start between two and 21 days of infection. The World Health Organization notes patients don't become infectious until symptoms begin developing.
Over 28,500 people in West Africa had Ebola between 2013 and 2016. More than one-third of them died.
The last of the West African countries at the heart of that Ebola outbreak was declared Ebola-free in June.
But the vaccine's manufacturer has stockpiled 300,000 doses, just in case another outbreak begins between now and when regulatory approval is granted.
One of the doctors involved with testing the vaccine told NPR the effectiveness of it is likely to drop from 100 percent once it's tested on more people.
The Public Health Agency of Canada and the U.S. Army developed the vaccine in 2005.