The trial enrolled 20 healthy volunteers between the ages of 18 and 50 in September. Ten volunteers received an intramuscular injection of vaccine at a lower dose and 10 received the same vaccine at a higher dose, the NIH said.

All 20 volunteers developed anti-Ebola antibodies within four weeks of receiving the vaccine and antibody levels were higher in those who received the higher dose vaccine, the agency said.

The investigators also found the vaccine prompted production of immune system cells called CD8 T cells, which may be an important part of immune protection against Ebola viruses.

"We know from previous studies in non-human primates that CD8 T cells played a crucial role in protecting animals that had been vaccinated with this NIAID/GSK vaccine and then exposed to otherwise lethal amounts of Ebola virus," said Julie Ledgerwood, the trial's principal investigator.

"The size and quality of the CD8 T cell response we saw in this trial are similar to that observed in non-human primates vaccinated with the candidate vaccine."

There were no serious adverse effects observed in any of the volunteers, although two people who received the higher dose vaccine did develop a briefly lasting fever within a day of vaccination, the NIH said.

The vaccine candidate is also being tested among healthy volunteers in Britain, Mali and Switzerland. Further data from these clinical trials are expected by the end of the year.

If the trials are successful, the next phases of the clinical trial program will begin in early 2015 which will involve the vaccination of frontline healthcare workers in the two of the affected countries, Sierra Leone and Liberia, GSK said in a statement.

Further safety studies will also be conducted in West African countries not affected by the current outbreak in adults and children, it added.

US President Barack Obama will visit the NIH next Tuesday "to congratulate the NIH team, mark this achievement, and discuss progress on other fronts on the fight against Ebola," the White House said in a statement, calling the results "another important milestone."