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.

The data released by the CDC is self-reported by pharmacies, states and other vaccine providers. It is not comprehensive — missing some states and federal providers — and it does not include the reason doses had to be thrown away. The number of discarded doses is still a small fraction of the total vaccines administered in the U.S.

In general, there are a number of reasons why vaccination sites may have to mark doses as wasted, from a cracked vial or an error diluting the vaccine to a freezer malfunction to more doses in a vial than people who want them. A wastage report can also happen when a vial contains fewer doses than it should.