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 human safety trials, which began in Geneva on Nov. 10, are due to resume on Jan. 5 in up to 15 volunteers after checks to ensure that joint pain symptoms in hands and feet were "benign and temporary", the hospital said. Fifty nine volunteers have been vaccinated so far.

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

On Thursday, it said that four patients had reported joint pains in the second week that had lasted a few days. 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 habitual and classic in all clinical trials," the researchers said.

The vaccine was originally developed at the National Microbiology Lab in Winnipeg and is licensed to NewLink and Merck.

The team was in close contact with researchers in the United States, Germany, Canada and Gabon who are carrying out the same trial with the Merck and NewLink vaccine, it said.

"These centres have not observed symptoms of inflammation in their volunteers to date," it added.

Marie-Paule Kieny, vaccine expert at the World Health Organization, told a news briefing that the delay would allow time to see how widespread the problems are. But it was expected that after the delay the trial will be able to continue as originally planned, she said.

"It's not a setback, not at all," Kieny said in Geneva.