One day after receiving her first dose of Moderna’s Covid vaccine, Luz Legaspi, 72, woke up with bruises on her arms and legs, and blisters that bled inside her mouth. She was hospitalized in New York City that day, Jan. 19, with a severe case of immune thrombocytopenia — a lack of platelets, a blood component essential for clotting. The same condition led to the death in January of Dr. Gregory Michael, 56, an obstetrician in Miami Beach whose...