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Officials from the Department of Health and Human Services and the Pentagon are holding a joint briefing Wednesday on the Trump administration's Covid-19 vaccine program Operation Warp Speed.

The briefing comes as the administration faces criticism for what appears to be a slower-than-expected rollout of the vaccines.

More than 11.4 million doses of Pfizer and Moderna's two-dose vaccines have been distributed across the country as of Monday morning, but just about 2.1 million shots have been given to people, according to the Centers for Disease Control and Prevention. That's a far cry from U.S. health officials' original goal of injecting at least 20 million Americans with their first shots before the end of the year.

The CDC, however, acknowledged delays in its vaccine data from the states and jurisdictions that collect and report it to federal officials, among other factors.

"A large difference between the number of doses distributed and the number of doses administered is expected at this point in the COVID vaccination program due to several factors," the agency said.

President-elect Joe Biden and public health specialists have criticized Trump's immunization program in recent days for failing to administer doses as quickly as they are distributed.

Read CNBC's live updates to see the latest news on the Covid-19 outbreak.