The Biden administration is expected to announce this week that it is purchasing 10 million courses of Pfizer’s covid pill, a multibillion-dollar investment in medication that officials hope will help change the trajectory of the pandemic by staving off many hospitalizations and deaths, according to two people with knowledge of the transaction. U.S. officials see this antiviral pill, and another by Merck and Ridgeback Biotherapeutics, as potential game-changers to help restore a broader sense of normalcy and are eager to add them to a small arsenal of treatments for Americans who contract the coronavirus. With breakthrough cases rising and 30...