The British approved the Pfizer vaccine, they approved the AstraZeneca vaccine, they moved to first doses first and now they are allowing (not yet encouraging they are running a trial) mix and match. Under the present circumstances, the British focus on doing what it takes to save lives is smart, admirable, and impressive.
As I wrote on Dec. 10, in Herd Immunity is Herd Immunity:
Mix and matching has two potentially good properties. First, mix and matching could make the immune system response stronger than either vaccine alone because different vaccines stimulate the immune system in different ways. Second, it could help with distribution. It’s going to be easier to scale up the AZ vaccine than the mRNA vaccines, so if we can use both widely we can get more bang for our shot.
Addendum: The CDC is projecting 80,000 COVID deaths in the United States over the next three weeks.