The first human trials for an Ebola vaccine have begun in Maryland.
Researchers are studying how human immune systems respond to the vaccine.
Twenty healthy adult volunteers are being tested at the National Institutes of Health (NIH) in Maryland.
The trial has been brought forward amid an outbreak of the Ebola disease in West Africa that has killed at least 1,500.
No volunteers will be infected with the Ebola virus but the vaccine has performed well in primate trials.
There is no cure for Ebola but a separate experimental drug, ZMapp, was 100% effective in studies on monkeys, according to researchers who published their data in Nature last week.
ZMapp has not been officially studied in humans, but it was used on seven people infected with Ebola in West Africa, mostly health workers.
Two of them later died and it is unclear if the drug helped the others to recover. The fatality rate during this outbreak has been about 50%, according to the World Health Organization (WHO).
At least 3,000 people have been infected with the virus. On August 29, a fifth country, Senegal, had its first confirmed case.
WHO has warned it could get much worse and infect more than 20,000 people.
“There is an urgent need for a protective Ebola vaccine, and it is important to establish that a vaccine is safe and spurs the immune system to react in a way necessary to protect against infection,” said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, which helped develop the drug.
Researchers in Bethesda, Maryland, will be looking for any adverse reactions and how each volunteer’s immune system responds to the vaccine.
NIH is also partnering internationally with the Wellcome Trust and others for a similar trial in the UK, the Gambia and Mali.