None of the other teams currently testing the vaccine has reported seeing the same side-effect, and the Swiss scientists acknowledge that the symptoms they have spotted are mild.
But they said Thursday that they want a little time to assess the situation before they resume vaccinating subjects in the clinical trial.
"We want to make sure that it doesn't move into something more severe before we inject anybody else," said Dr. Angela Huttner in an interview from Geneva.
"So far all signs are really positive."
Four of 59 people vaccinated to date in the Geneva-based trial reported pain in the joints of their extremities — fingers and toes — between 10 and 15 days after receiving the shot. Investigation of a possible fifth case is underway.
None of them needed medical care and Huttner said the team believes the symptoms will probably disappear with time. "(But) we want to make sure these are truly transient.''
The vaccine was designed by scientists at Canada's National Microbiology Laboratory in Winnipeg and donated to the World Health Organization by the Canadian government.
The licence for the vaccine was recently acquired by pharmaceutical giant Merck from NewLink Genetics, a small Iowa-based biotech company that had been developing the experimental vaccine.
Huttner said the research team contacted NewLink and Merck, as well as the World Health Organization to inform them of the developments.
The Geneva trial is testing two different but relatively high doses of the product. Experts hope a lower dose would be effective and perhaps trigger fewer side-effects.
The clinical trial was set to take a two-week break over the holidays and the research team decided to add a week to that hiatus to give themselves time to monitor the people who have been vaccinated so far and investigate what is going on.
"The reason we want to hold just for a few weeks is because this wasn't expected. The other sites aren't seeing this. We just want to know what's going on before we do any more injections," Huttner said.
Another trial of the vaccine, which is known as rVSV-ZEBOV, is being conducted at the Canadian Center for Vaccinology in Halifax. The centre said Thursday that it is aware of the developments in Geneva but is continuing to vaccinate.
A spokesperson for the centre said the principal investigator of the Canadian trial, Dr. Scott Halperin, has reviewed the data gathered from the Halifax volunteers so far and has seen no concerning side-effects.
"It is important to note that the vaccine dose in the Geneva trial is three times higher than the highest dose used in the Canadian trial which is testing three different doses of the vaccine," Mary Appleton said in an email.
Other clinical trials of this vaccine are being conducted at the U.S. National Institutes of Health and the Walter Reed Army Institute of Research, both in Bethesda, Md., in Germany and in Gabon. Another is set to start soon in Kenya.
Dr. Marie-Paule Kieny, who is leading the WHO's efforts to spur development of Ebola drugs and vaccines, said four of the trials share a data safety monitoring board — the ones in Geneva, Germany, Gabon and Kenya. None of those four will use doses in the range used in the Geneva trial until the Geneva team resumes its operations, she said.
In some clinical trials volunteers receive either an active drug of vaccine or a placebo. But Huttner said it is known that all four reporting joint pain in this trial received vaccine.
Three received the lower of two doses being tested in Geneva; the dose the fourth person received isn't currently known. That person is taking part in a portion of the trial that is "blinded" — which means the researchers will only learn after they analyze the trial's results which dose he or she received.
The trial is schedule to resume on Jan. 5, the University Hospitals of Geneva — where the research is taking place — said in a press release.
Huttner, who is an infectious diseases specialist, said people can experience joint pain after having a viral infection and after receiving some vaccines. It is commonly seen in women who are vaccinated against rubella, she said.
She said the team thinks this side-effect is likely an acceptable one but wants to see how common it is and how severe it might be. A number of the people who have received the vaccine in the Geneva trial are not yet at Day 15 after their vaccination.
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