As of this report, there are 23 coronavirus vaccines being tested in humans.
In order for researchers to determine if the vaccines are effective, people who have been vaccinated need to be exposed to the coronavirus to see if they are protected. Researchers cannot deliberately expose people, so the vaccines are tested in places where the virus is circulating at higher levels in the community.
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If a large number of participants are naturally exposed, researchers could have their answer fairly quickly. However, if that doesn’t happen there are people signing up to step in and volunteer to become exposed. They are agreeing to participate in “challenge trials.”
Right now, researchers believe they can test the virus vaccines without these challenge trials due to the number of hot spots around the country and world.
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