Vials of the Pfizer-BioNTech Covid-19 vaccine at the Sun City Anthem Community Center vaccination site in Henderson, Nevada, U.S., on Thursday, Feb. 11, 2021.

The United States has thrown away at least 15.1 million doses of Covid-19 vaccines since March 1, according to government data obtained by NBC News — a number that is far larger than previously known and is still likely an undercount.

Four national pharmacy chains reported more than 1 million wasted doses each, according to data released Tuesday by the Centers for Disease Control and Prevention in response to a public records request. Walgreens reported the most waste of any pharmacy, state or other vaccine provider, with nearly 2.6 million wasted doses. CVS reported 2.3 million wasted doses, while Walmart reported 1.6 million and Rite Aid reported 1.1 million.