The World Health Organization said 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 to develop immunity.

WHO Director General Margaret Chan said Friday that the preliminary results are promising.

“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.

While the initial results are promising, more 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 killed over 11,000 in Guinea, Liberia and Sierra Leone, a WHO report says.