Governments and companies are setting up websites where people can sign up.

Enthusiasm is high: More than 400,000 people have signed a registry of possible volunteers that's part of a vaccine network set up by the U.S. National Institutes of Health.

But before raising your hand, it's important to understand how the research works.

Initial studies include only a few dozen young, healthy volunteers, since this is the first chance to see if a shot causes a risky reaction in people. Older adults, anyone with a serious underlying illness, and pregnant women are typically excluded from this testing...