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CNN  — 

President Donald Trump is expected to sign an executive order on Tuesday aimed at prioritizing the shipment of the coronavirus vaccine to Americans before other nations, according to senior administration officials.

The signing is slated to take place during a vaccine summit at the White House, with the hope that the order will allay fears that there will not be enough doses of the vaccine to go around after distribution begins.

White House officials described the order as a “reaffirmation of the President’s commitment to America first.” But on a call with reporters Monday evening, in which administration officials asked not to be identified publicly, the White House did not provide specifics on how the order will do that.

The order comes after The New York Times reported Monday that Pfizer had offered to sell the US government additional doses in late summer, but the newspaper said the administration had turned it down. The decision to not purchase more doses mounted administration concerns that Pfizer would be unable to fulfill any additional US order until June because of the company’s commitments to other countries.

On the Monday evening call, Trump administration officials denied they had turned down an opportunity to buy more doses of the Pfizer vaccine months ago. One senior administration official said the administration is “in the middle of a negotiation right now” and can’t speak publicly about the Pfizer talks.

“But we feel absolutely confident” that there will be a “sufficient number of doses to vaccinate all Americans who desire one before the end of the second quarter of 2021,” the official said.

The initial US contract, an advance purchase agreement with Pfizer, was signed on July 22, the administration official said. That agreement was for 100 million doses with the option to purchase more. The 100 million doses would be enough to vaccinate 50 million people, because it’s a two-dose vaccine.

The administration official said on Monday’s call that they are continuing to negotiate with several companies working on Covid-19 vaccines.

The US has the opportunity to purchase 3 billion doses of vaccine, among all the contracts the federal government currently has with various companies. That would in theory be enough to vaccinate the US population several times over.

Pfizer would not disclose whether a deal for additional doses for the US had fallen through or if any additional orders would be unable to be fulfilled until the summer.

“(The) U.S. government placed an initial order of 100 million doses for Pfizer-BioNTech’s COVID-19 vaccine and Pfizer is ready to begin shipping initial doses soon after receiving an Emergency Use Authorization from FDA,” the company said in a statement. “Any additional doses beyond the 100 million are subject to a separate and mutually-acceptable agreement. The company is not able to comment on any confidential discussions that may be taking place with the U.S. government.”

Administration officials on Monday evening’s call also confirmed that representatives for Pfizer and Moderna will not attend Tuesday’s White House summit, despite being the companies behind the coronavirus vaccines seeking emergency use authorization by the Food and Drug Administration.

White House officials said representatives for the companies won’t be there “because the administration felt it was more important for regulators to attend and explain the authorization process to the American people. “

Both companies were involved with “initial discussions” of the planning of the summit, but “there was a change of direction in light of the fact we would have the regulator participating in the event,” an official said.

“It was more appropriate not to add one or more vaccine companies with pending applications before the FDA,” the official said.

Dr. Peter Marks, the FDA official in charge of which Covid-19 vaccines get authorized, will attend to “explain to the audience how the FDA goes about reviewing vaccines, and how thorough they are – just why the FDA review process is the gold standard for the world,” an official described on the call.

An official billed the summit as an effort to educate Americans about the vaccine approval process, assure that the vaccine is safe and to “congratulate the members of Operation Warp Speed.”

Also absent from the list of participants is Dr. Anthony Fauci, the head of the National Institute of Allergy and Infectious Diseases. An official said Fauci was invited and wanted to participate but wasn’t able to because of an “important scheduling issue.”

CNN’s Amanda Sealy and Samira Said contributed to this report.