Covid Live Updates: Mixing Vaccines Provides Strong Immunity, Study FindsPfizer and AstraZeneca work together well in a two-dose vaccination schedule; so do three doses of AstraZeneca, researchers find in separate studies.Covid Live Updates: Mixing Vaccines Provides Strong Immunity, Study Finds
Scientists Report Earliest Known Coronavirus Infections in Five U.S. StatesBlood drawn from nine people in the earliest days of the pandemic tested positive for the infection. But some experts questioned the results.Scientists Report Earliest Known Coronavirus Infections in Five U.S. States