Testing of an Ebola vaccine has been suspended until Jan. 5 after four patients given the drug in a trial complained of joint pain in their hands and feet.
A spokesperson close to the trials said checks are being conducted to make sure the symptoms are "benign and temporary."
Meanwhile, the delay is not worrying scientists, who say suspending trials is common in human drug testing.
"It's not a setback, not at all," Marie-Paule Kieny, vaccine expert at the World Health Organization, told reporters in Geneva.
She explained that the delay to the trial would allow time to see how widespread the problems are, but the trial should then be able to continue as originally planned.
Scientists are racing to approve a vaccine by June. A vaccine may be the only effective measure in controlling the Ebola outbreak in West Africa, which has claimed 6,000 lives so far.
Aid efforts continue, but the lack of adequate education and medical facilities is making the Ebola outbreak nearly impossible to contain.
The symptoms reported by volunteers are from a study being conducted in Geneva of an Ebola vaccine licensed by the pharmaceutical company Merck.
"They are all fine and being monitored regularly by the medical team leading the study," a hospital spokesperson in Geneva said.
Meanwhile, another Ebola vaccine, developed by the pharmaceutical company GlaxoSmithKline is being tested on humans in Lausanne, Switzerland, and is progressing without major incident.
Professor Blaise Genton, who is leading the GSK trial of 120 volunteers in Lausanne, said the results so far are "satisfactory."
"General symptoms such as fever might be slightly more frequent," he commented, "though no serious adverse event has been observed so far."
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.
Once an Ebola vaccine is approved for mass distribution it can be implemented. GAVI, the global vaccines alliance, pledged $300 million on Thursday to buy Ebola vaccines.
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