There is currently no licensed treatment or vaccine for Ebola, which has so far killed more than 11,000 people in West Africa since the world's biggest outbreak began in the forest region of Guinea last year. Cases have dropped dramatically in recent months in the other two hard-hit countries, Sierra Leone and Liberia.

"If proven effective, this is going to be a game-changer," said Dr. Margaret Chan, Director-General of the World Health Organization, which sponsored the study. "It will change the management of the current outbreak and future outbreaks."

Scientists have struggled for years to develop Ebola treatments and vaccines but have faced numerous hurdles, including the sporadic nature of outbreaks and funding shortages. Many past attempts have failed, including a recently abandoned drug being tested in West Africa by Tekmira Pharmaceuticals.

The study involved several thousand people who had been near a new Ebola patient or a close contact of one. They were randomly assigned to get the vaccine right away or in three weeks.

Researchers started tracking results 10 days after they set up the groups to give time to weed out any people who might have been silently harboring the virus when the study began.

After that point, none of the people in the group that had been assigned to get the vaccine right away developed Ebola, versus 16 people in the group eligible to get the vaccine after 21 days.

The vaccine, developed by the Canadian government, has since been licensed to Merck & Co. but has not yet been approved by regulators. The study results were published online Friday in the journal Lancet.

At the moment, officials think the vaccine would only be used once an outbreak starts, to protect those at high-risk; there are no plans to introduce mass vaccination campaigns like those for measles or polio or to create huge stockpiles of the shots.

Merck, based in Kenilworth, New Jersey, noted its vaccine is in what is normally the final round of human testing in Sierra Leone, and in mid-stage testing in Liberia.

Merck will manufacture the vaccine if it's approved for use outside patient studies. In late-morning trading in the U.S., Merck shares were up 62 cents, or 1.1 percent, at $59.13.

Last December, Gavi, the vaccine alliance, said it would spend up to $300 million buying approved Ebola vaccines. The private-public partnership, which often buys immunizations for poor countries, said Friday that it "stands ready to support the implementation of a WHO-recommended Ebola vaccine."

An expert group monitoring the study's data and safety recommended the trial be stopped on July 26 so that everyone exposed to Ebola in Guinea could be immunized.

The vaccine uses an Ebola protein to prompt the body's immune system to attack the virus.

"It looks to be about as safe as a flu vaccine," said Ben Neuman, a virologist at the University of Reading who was not part of the trial. Researchers are still assessing possible side effects; the most serious seemed to be fever and the stress experienced by patients who believe such symptoms were due to Ebola.