The U.S. Food and Drug Administration warned health care personnel not to make any changes to the dosing of Covid-19 vaccines and that doing so would place the public health at risk and undermine “the historic vaccination effort to protect the population.”

The Centers for Disease Control and Prevention administered 4,563,260 first doses of Covid-19 vaccines in the United States as of the morning of Jan. 4 and distributed 15,418,500 doses.