DALLAS—Ebola vaccines are so effective in monkeys that macaques can be protected or rescued even if they're injected with a hundred times the lethal dose of the Ebola virus after vaccination. But no one knows for certain whether the vaccines will work in humans; the vaccines haven't yet been rigorously tested in people.

Just developing the vaccines to test in monkeys was a grueling, decades-long process that has killed scores of macaques since the 1990s. American researchers almost gave up. "No one thought you could protect with a vaccine, because [Ebola] was so aggressive," says Nancy Sullivan, a top official at the U.S. National Institute of Allergy and Infectious Diseases.

The insidious nature of the Ebola virus has been among