Pharmacies and state governments in the United States have thrown away at least 15.1 million doses of Covid-19 vaccines since March 1, according to government data obtained by NBC News — a far larger number than previously known and still probably 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.

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. In one example of missing data, the CDC lists just 12 wasted doses for Michigan since March, but Michigan’s Department of Health and Human Services said on Wednesday that the state has thrown away 257,673 doses since December.

The number of discarded doses is still a small fraction of the total doses 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.

The data on wasted doses comes as the more contagious delta variant spreads rapidly across the United States, adding fresh urgency to the effort to vaccinate as many people as possible and spurring a plan to begin offering booster shots to those already vaccinated — even as many nations around the world have vaccinated few, if any, of their residents.

“It’s really tragic that we have a situation where vaccines are being wasted while lots of African countries have not had even 5 percent of their populations vaccinated,” said Sharifah Sekalala, an associate professor of global health law at England’s University of Warwick, who studies inequalities in infectious diseases.

“A lot of the global south is unvaccinated. The African continent is still below 10 percent, and that’s just a huge inequality and it’s really problematic.”

CDC spokeswoman Kristen Nordlund said in an email that the share of Covid vaccines wasted “remains extremely low, which is evidence of the strong partnership among the federal government, jurisdictions, and vaccine providers to get as many people vaccinated as possible while reducing vaccine wastage across the system.”

Nordland added, “As access to Covid-19 vaccine has increased, it is important for providers to not miss any opportunity to vaccinate every eligible person who presents at vaccine clinics, even if it may increase the likelihood of leaving unused doses in a vial.”