Pfizer Pharmaceuticals on Tuesday asked the federal government to authorize extra-low doses of its COVID-19 vaccine for children under five. If approved, this would open the way for the very youngest Americans to start receiving shots as early as March. The Food and Drug Administration had urged Pfizer to apply earlier than the companies had planned, and before it’s settled, the youngsters will need two shots or three. The nation’s 19 million children under five5 are the only group not eligible for a vaccination against the coronavirus.