WASHINGTON — Three potential COVID-19 vaccines are making fast progress in early-stage testing in volunteers in China and the U.S., but it’s still a long road to prove if they’ll really work.

China’s CanSino Biologics has begun the second phase of testing its vaccine candidate, China’s Ministry of Science and Technology said Tuesday.

In the U.S., a shot made by the National Institutes of Health and Moderna Inc. isn’t far behind. The first person to receive that experimental vaccine last month returned to a Seattle clinic Tuesday for a second dose.