The Geneva University Hospital (HUG) announced the same day that it was suspending trials of one of two experimental vaccines being tested on humans in several countries, after several volunteers experienced unexpected joint pains.
But WHO assistant director general Marie-Paule Kieny said the hospital's decision to suspend the trials of the VSV-ZEBOV vaccine a week ahead of plan should not be considered a big deal.
"It's not a setback," she told reporters in Geneva.
There is no licensed treatment or vaccine for Ebola, and the WHO has endorsed rushing potential ones through trials in a bid to stem the epidemic, which has killed nearly 6,400 people mainly in Guinea, Liberia and Sierra Leone.
The Geneva trials of the experimental vaccine, manufactured by the Public Health Agency of Canada and licensed by US firm NewLink Genetics began on November 10th, are still expected to resume next month.
HUG said its decision came after four of the 59 volunteers taking part in the trial had experienced minor joint pains in their hands and feet ten to 15 days after receiving the shot.
The pains had lasted a few days, the hospital said, pointing out that although joint pain had not figured among the expected side-effects of the potential vaccine, it was common after other kinds of vaccines.
"This phenomenon is very well-studied and not of concern to specialists," it said.
Kieny, who is in charge of WHO's Ebola vaccine quest, said that none of the affected volunteers had needed to be hospitalized.
The early suspension would allow an in-depth analysis of the vaccine, with the aim of resuming trials as planned on January 5th.
"The investigators are looking at how frequently this happens in order to be able to document the future safety of the use of the vaccine," Kieny said.
The VSV-ZEBOV vaccine is also being tested on volunteers in the United States, Canada, Germany and Gabon, and clinical trials are also due to start soon in Kenya.
Joint pain has not been reported from any of the other trial sites, Kieny said.
Another experimental vaccine, ChAd3, made by Britain's GlaxoSmithKline, is being tested on another set of volunteers at another Swiss hospital, the CHUV in Lausanne.
Trials of that vaccine are also underway in Mali, Britain and the United States.
At talks in Geneva on Thursday, health chiefs from Sierra Leone, Guinea and Liberia expressed their eagerness to have trials conducted in their Ebola-hit countries.
Meanwhile, at Lausanne's university hospital (CHUV), doctors said 120 volunteers "tolerated rather well" an Ebola virus vaccine being tested there.
The volunteers had largely experienced the same kind of reactions normally found with other vaccines, the hospital said, according to a report from Le Matin.
"The security data is up to now staisfactory," Professor Blaise Genton, medical doctor in charge of infectious diseases at CHUV said in a news release.