In the first results from tests on an experimental Ebola vaccine, researchers at the National Institute of Allergy and Infectious Diseases (NIAID) report for the first time Wednesday that the shot is safe and that it leads to an immune response among healthy volunteers. The vaccine, developed by the National Institutes of Health and GlaxoSmithKline, was tested in 20 participants in the US at the NIH Clinical Center in Bethesda.

“This tells us that this is kind of a positive signal about moving to the next stage,” says Dr. Anthony Fauci, director of NIAID.

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The vaccine is meant to protect uninfected people from Ebola, and, if effective, would be tested next in populations in high-risk areas such as west Africa, where the outbreak is ongoing, to immunize them against the virus. It does not contain live Ebola virus, but does contain snippets of its protein coat, just enough to alert the immune system to produce antibodies and immune cells that can recognize and destroy any live viruses that people might eventually encounter. The pieces of Ebola protein are introduced to the body via another virus, one related to the common cold that infects chimps.

The shot was tested in two doses among 10 people each; it triggered antibodies and immune cells that in primate studies were enough to protect them against a challenge of Ebola up to 10 months later. Whether that’s also the case among people won’t be known until the vaccine is tested among thousands more in west Africa, including health care workers and family members of ill patients, who are most vulnerable to getting exposed to the virus.

Only two people reported brief fevers after getting vaccinated.

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The vaccine is one of two that scientists are preparing to test in the outbreak zone beginning in early 2015. Another candidate uses an entirely different approach involving a live Ebola virus that has been crippled so it can’t reproduce once it infects a person. The advantage to that system is that it might launch a stronger and more robust immune response than the NIAID and GSK shot, which may require a boost after several months to maintain strong immune protection against Ebola. “At the end of the day, you don’t know which is the most effective until you test them, and that’s the point,” says Fauci.

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