A clinical trial of an Ebola vaccine developed by Merck and NewLink has been halted temporarily as a precautionary measure after four patients complained of joint pains, the University of Geneva Hospital said on Thursday.

"They are all fine and being monitored regularly by the medical team leading the study," it said in a statement.

The trials will resume on Jan. 5, on up to 15 volunteers, after checks to ensure that the joint pain symptoms in hands and feet were "benign and temporary", the hospital said.

Fifty nine volunteers have been vaccinated so far in the human safety trials in Geneva, which began on Nov. 10.

Scientists are racing to develop Ebola vaccines after the world's worst outbreak of the virus has killed more than 6,000 people in West Africa so far this year.

Separately, safety data from a trial of a GlaxoSmithKline Ebola vaccine on 120 volunteers is "satisfactory", the University of Lausanne Hospital said on Thursday.

The first results of the Lausanne hospital's trial of the GSK vaccine and whether it provides immunity against the virus are expected by the end of December, the university said in a statement.

"The safety data looked satisfactory so far," said Professor Blaise Genton, who is leading the GSK trial in Lausanne. "General symptoms such as fever might be slightly more frequent, though no serious adverse event has been observed so far."

Scientists reported on Nov. 26 in the New England Journal of Medicine that another version of the experimental GSK vaccine caused no serious side effects and produced an immune response in all 20 healthy volunteers who received it in an early-stage trial.

The Geneva researchers reported on Dec. 2 that the first people vaccinated with the Merck-NewLink shot had seen no serious side effects, but a few had mild fever.

On Thursday, the team said that four patients had reported joint pains in the second week that had lasted a few days. Before it was suspended, this first phase of the trial had been due to continue for another week.

"The Geneva team has decided to allow time to understand what is happening. This precaution of momentarily suspending the trial is usual and classic in all clinical trials," the team said.

It was in close contact with researchers in the United States, Germany, Canada and Gabon who are carrying out the same trial on the Merck-NewLink vaccine, it said. "These centers have not observed symptoms of inflammation in their volunteers to date."

Marie-Paule Kieny, vaccine expert at the World Health Organization, said that the delay to the Merck-NewLink trial would allow time to see how widespread the problems are, but the trial should then be able to continue as originally planned.

"It's not a setback, not at all," Kieny told a briefing.

GAVI, the global vaccines alliance, pledged $300 million on Thursday to buy Ebola vaccines.