The world's biggest COVID-19 vaccine test got underway Monday with the first of 30,000 planned volunteers.
The experimental vaccine is made by the National Institutes of Health and Moderna Inc., and it's one of several candidates in the final stretch of the global vaccine race.
The needed proof: Whether more people who get dummy shots become infected than those given two doses of the real vaccine.
The U.S. government plans separate huge studies of several leading vaccine candidates through fall, each in hot spots where the virus still is spreading.