A trial in Switzerland of a Canadian-made Ebola vaccine resumed this week after being suspended over concerns about an unexpected side-effect reported by some volunteers.

And while the Geneva-based trial of the rVSV-ZEBOV got back on track, researchers at Oxford University began testing another Ebola vaccine, made by the Janssen Pharmaceutical Companies of Johnson & Johnson.

The Janssen vaccine joins rVSV-ZEBOV, which is being developed by Merck and NewLink Genetics, and a vaccine being developed by GSK (formerly GlaxoSmithKline) in the race to provide the world with a licensed Ebola preventative.

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The Swiss trial of rVSV-ZEBOV was temporarily suspended in mid-December after several participants reported experiencing pain in the joints of their fingers or toes about two weeks after being injected with the vaccine. It has been resuming using a substantially lower dose of the vaccine than what was given earlier.

A two-week Christmas break had been planned for the trial, but that hiatus – stretched to three – was started early so the research team could investigate the reaction.

At the time the trial was halted four participants had complained of experiencing joint pain. Dr. Angela Huttner, one of the researchers heading the trial, said more cases came to light, but she would not reveal how many of the 59 volunteers enrolled in the study before the break reported joint pain.

"The number did go up," she said on Tuesday.

During the pause the researchers studied a number of the people who reported joint pain to see if they could find another explanation for the problem. But in the end they concluded the pain was likely a side-effect of the vaccine. The study's informed consent form was altered to warn future volunteers they might experience joint pain.

"We did very extensive work-ups to rule out other causes, to rule out infections and any other kinds of things that can bring on these kinds of joint symptoms," Ms. Huttner said. "We do feel that the joint symptoms were linked to the vaccine."

Most of the complaints related to pain in fingers and toes, but some people reported pain in their knees, she acknowledged. And while it had not been expected as a side-effect for this vaccine, Ms. Huttner and her colleagues pointed out that joint pain is a known side effect of other live-virus vaccines.