British drugmaker AstraZeneca said Monday it has started Phase 3 trials of its experimental coronavirus vaccine in the United States, becoming the third company to start late-stage trials of a vaccine to prevent COVID-19.The vaccine, developed in partnership with Oxford University, has the backing of the U.S. federal government. Rivals Moderna and Pfizer/BioNTec already have Phase 3 trials under way, also with federal government funding.AstraZeneca