The first human trials of a vaccine against the deadly Ebola virus will start next week in the U.S., officials announced Thursday, just as the World Health Organization predicted as many as 20,000 people could be infected in West Africa before the epidemic is brought under control.

The National Institutes of Health will sponsor the first trial of the vaccine, one of several being developed against Ebola. It’s fast-tracked the testing because of the outbreak of Ebola that is ravaging three West African countries.

“We have the green light to begin,” said Dr. Tony Fauci, head of the U.S. National Institute for Allergy and Infectious Diseases.

The vaccine is being developed by NIAID and drugmaker GlaxoSmithKline. “Testing will take place at the NIH Clinical Center in Bethesda, Maryland,” NIH said in a statement.

Ebola has killed more than 1,500 people out of more than 3,000 infected in Liberia, West Africa and Guinea in the ongoing outbreak, by far the worst outbreak of Ebola ever seen. And the WHO says those numbers almost certainly understate the true numbers of those infected and killed. WHO predicted on Thursday that as many as 20,000 could become infected.

The trial will enroll 20 healthy adults who will get a shot in the arm to see first if the vaccine is safe, and second if it generate an immune response that should, in theorym protect against Ebola infection.

It’s the first in a series of vaccine trials that will roll out this fall. NIH with British medical foundations will spearately test the vaccine in Britain, Gambia and Mali, and the Centers for Disease Control and Prevention is discussing a trial in Nigeria.

NIAID has been working on an Ebola vaccine for years. The idea was to develop it to defend people in case Ebola or a related virus, Marburg, was ever used in a biological attack. Previous outbreaks of the virus were always too small and too easily controlled to justify developing a vaccine quickly.

NIAID was working with a small Swiss-Italian biotech company called Okairos to develop the vaccine. It’s been shown to protect monkeys against Ebola. Glaxo bought the company last year.

Glaxo’s Dr. Rip Ballou said NIH asked the company if it could accelerate development of one of the vaccines it was working on.

“Now with this outbreak in West Africa that looks unprecedented, the WHO has requested that we think about developing a vaccine that could be used and licensed in the region and possibly stockpiled for future outbreaks,” Ballou told NBC News.

“This is very new for us. We are only beginning to think about how this could be done.”

Although the vaccine works well in monkeys, it’s important to be sure it’s safe in people, Fauci and Ballou both stressed. People sick with Ebola can’t afford to be made even sicker.

“Safety is paramount. It is absolutely paramount,” he told reporters. “I have been fooled enough in my many years experience in infectious disease vaccinology,” he added. “You really can’t predict what you might see. The worst thing you want to do is let something widely out before you have even tested a modicum of safety.”

The vaccine is made using a virus called an adenovirus that infects chimpanzees but not people. The virus is genetically engineered with a single piece of Ebola virus, a protein that the immune system can recognize, but which doesn’t make people sick. It protects against two of the strains of Ebola virus, including the Zaire strain that’s affecting West Africa now.

The chimp virus was chosen as a vehicle for the vaccine because it shouldn’t make people sick. “It does not cause disease in humans. It does not normally infect humans,” Ballou said. That also means the body’s immune system shouldn’t attack the vaccine before it can do its work. “People don’t already have antibodies against it,” he said.

Several other companies are working to develop Ebola vaccines, including Crucell, a small biotech called Profectus Biosciences, Iowa-based NewLink Genetics and Immunovaccine Inc, based in Nova Scotia, Canada. Trials are being prepared for some of those vaccines, also.

Two American medical missionaries, Dr. Kent Brantly and Nancy Writebol, were treated with an experimental therapy made by California-based Mapp Biopharmaceutical. Three Liberian doctors also received the drug. One died and the other two have recovered.

WHO published what it called a road map on Thursday for fighting the outbreak.

“Clearly, a massively scaled and coordinated international response is needed to support affected and at-risk countries in intensifying response activities and strengthening national capacities,” it says.

“Response activities must be adapted in areas of very intense transmission and particular attention must be given to stopping transmission in capital cities and major ports, thereby facilitating the larger response and relief effort.”

This outbreak is different from other outbreaks across Africa over the past 40 years, it says.

“This Roadmap assumes that in many areas of intense transmission the actual number of cases may be 2-4 fold higher than that currently reported,” it says.

None of the provisions call for vaccination – it’s far to soon for that. Instead, the plan calls for better tracking down and diagnosis of cases, a fast-track training program to get more health workers on the job, safer burials and better control of travelers.

“Today we know the best way to prevent the spread of Ebola infection is through public health measures, including good infection control practices, isolation, contact tracing, quarantine, and provision of personal protective equipment,” Fauci said in a statement. “However, a vaccine will ultimately be an important tool in the prevention effort. The launch of Phase 1 Ebola vaccine studies is the first step in a long process.”