Two potential Ebola vaccines are currently being tested in people, to see if they're safe and to figure out the best dose.

Both trials have encountered some of the typical travails of vaccine research.

One vaccine, which uses a modified chimpanzee virus, was associated with some mild fevers at its highest dose. And today, the University Hospitals of Geneva said that four healthy volunteers receiving a second potential vaccine, called VSV-ZEBOV, had experienced unexpected joint pain.

The hospital has already given the vaccine to 59 people, and was about to inject another 15. But researchers there decided to hold off on those next 15 people for a week or so, while they pause to take a closer look at the people who experienced mild pain in their hands and feet. All told, the Geneva hospital plans to inoculate 115 volunteers.

Despite this side effect, "Initial results show that the vaccination is very well tolerated," the hospital said in a statement.

The hospital says joint pain is a common side effect of vaccines, but it had not listed this as a potential side effect for this particular study. The hospital says it will update its list of potential side effects before proceeding.

The vaccine was developed by the Canadian government and then licensed to a small U.S. biotech company called , which teamed up with pharmaceutical giant Merck for testing, manufacturing and distribution. Merck told NPR that the vaccine will continue to be tested at other sites using lower doses.

Three major manufacturers have been pursuing Ebola vaccines, and the World Health Organization says Russia has something in the works as well. In the best of circumstances, it will take many months of study to figure out whether the vaccine is safe and effective. But assuming one or more of these vaccines can protect people from Ebola, GAVI the Vaccine Alliance, says it's ready to commit $300 million to buy vaccines for Africa.

GAVI is a public-private partnership that includes U.N. agencies, the World Bank, foundations and private companies. "Up to an additional $ 90 million could be used to support countries to introduce the vaccines and to rebuild devastated health systems and restore immunisation services for all vaccines in Ebola-affected countries," Gavi said in a statement.

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