The World Health Organization (WHO) announced Friday that an Ebola vaccine has shown great promise in halting the spread of the deadly virus during a clinical trial in Guinea.

"We believe that the world is on the verge of an efficacious Ebola vaccine," Marie-Paule Kieny, WHO assistant director general for health systems and innovation, said in announcing the results of a preliminary study on the vaccine trial.

The vaccine, VSV-EBOV, was developed by the Public Health Agency of Canada and is licensed to Merck. The new vaccine contains no live Ebola virus. Instead, it deploys a different virus, one that is alive and replicating, and has been modified to replace one of its genes with a single Ebola virus gene. The result is that the body's immune system has an Ebola-specific response and is better able to fight off an Ebola infection.

According to results published in the journal Lancet on Friday, the vaccine was found to be 100 percent effective in people treated. More than 4,000 people have been vaccinated with VSV-EBOV, and none has developed Ebola after between six and 10 days, the amount of time needed for people to develop immunity.

WHO Director General Margaret Chan said Friday that the preliminary results are promising; if confirmed, she said, it "is going to be a game-changer."

"It will change the management of the current Ebola outbreak and future outbreaks," she said.

The trial began in March in Ebola-affected communities in Guinea. Using a "ring" vaccination method, researchers selected people around an infected person to create a circle of protection. The method had been used in the 1960s and 1970s to help eradicate smallpox. It is designed to both protect potential contacts and help halt the spread of the virus.

While the initial results are promising, additional research is needed to determine whether the protective effect of the vaccine will remain over a long period of time.

"There are still some questions about its broad applicability over time," Anthony Fauci, director of NIH National Institute of Allergy and Infectious Diseases, said in an interview. "The study wasn't designed to determine durability. It was designed to determine was it efficacious."

Fauci said the question is whether this vaccine will continue to provide protection against Ebola infection in, say, six months. The trial by design tests only whether it is effective in the short term. Further studies will determine if this vaccine works over the long term.

Still, he called the results "very impressive."

The Ebola outbreak has left more than 11,000 dead in Guinea, Liberia and Sierra Leone, according to a WHO situation report released Wednesday.

"This is Guinea's gift to West Africa and the world," Sakoba Keita, Guinea's national coordinator for the Ebola response, said in a statement about the vaccine. "The thousands of volunteers from Conakry and other areas of Lower Guinea, but also the many Guinean doctors, data managers and community mobilizers have contributed to finding a line of defense against a terrible disease."

Initially, half of the study's participants were randomly selected to receive the vaccine three weeks after an infected contact was identified in order to provide a comparison. The results reported in the Lancet cover research conducted between April 1 and July 20.

Because of the promising results, researchers stopped using a randomized method to determine when people received the vaccine and began vaccinating at-risk people immediately on Sunday. The trial will be expanded to include adolescents and possibly children, if there is evidence that the vaccine is safe for those populations.

"I am very excited by it; I think it is remarkable," said Donald Henderson, a professor of epidemiology at Johns Hopkins Bloomberg School of Public Health who consulted on the design of the Ebola vaccine trial. "It is a single-dose vaccine and the results are more than I could possibly have expected."

Henderson, who is known for his work on the eradication of smallpox, said Friday that "the real problem that we had with all of the vaccines, it was just difficult to figure out how you could possibly do a controlled trial in the situation Guinea was in."

A traditional placebo trial, he added, "did not seem to be a great idea for a lot of reasons. It's the business of saying 'You are getting the drug' and 'You are not getting the drug.' ... How can you possibly make it available to some and not to others?"

So, Henderson said, researchers settled on a study design in which everyone who had contact with an Ebola patient would get the still-unproven vaccine candidate eventually -- but some people, chosen randomly, would not get it for 21 days.

"For the first time ever, we received evidence of efficacy of a vaccine that will help fighting Ebola," Bertrand Draguez, of Doctors Without Borders, said in a statement. "Too many people have been dying from this extremely deadly disease, and it has been very frustrating for health-care workers to feel so powerless against it."

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Washington Post staff writer Amy Brittain contributed to this report.