WASHINGTON (AP) — President Donald Trump's administration is facing new scrutiny Tuesday after failing to lock in a chance to buy millions of additional doses of Pfizer's coronavirus vaccine, which has shown to be highly effective against COVID-19.

That decision could delay the delivery of a second batch of doses until Pfizer fulfills other international contracts.

The revelation comes as Trump's plans to host a White House summit aimed at celebrating the expected approval of the first vaccine later this week. His administration is seeking to tamp down public skepticism over the vaccine and secure a key component of the Republican president's legacy.

The focus was to be on the administration’s plans to distribute and administer the vaccine, but officials from President-elect Joe Biden’s transition team, which will oversee the bulk of the largest vaccination program in the nation’s history once he takes office Jan. 20, were not invited.

Pfizer's vaccine is expected to be endorsed by a panel of Food and Drug Administration advisers as soon as this week, with delivery of 100 million doses — enough for 50 million Americans — expected in coming months.

Under its contract with Pfizer, the Trump administration committed to buy an initial 100 million doses, with an option to purchase as many as five times more.

But this summer, the White House opted not to lock in an additional 100 million doses for delivery in the second quarter of 2021, according to people who spoke on condition of anonymity because they were not authorized to discuss it publicly.

Dr. Moncef Slaoui, who is leading the government’s vaccine effort, noted the Trump administration was looking at a number of different vaccines during the summer. He told ABC's “Good Morning America” on Tuesday that “no one reasonably would buy more from any one of those vaccines because we didn’t know which one would work and which one would be better than the other.”

Adm. Brett Giroir, assistant secretary for health administration, told “CBS This Morning” the U.S. expects to be able to vaccinate about 20 million people this month, 20 million to 25 million in January and another 20 million to 25 million in February with the Pfizer vaccine and another vaccine from drugmaker Moderna, assuming both receive FDA emergency authorization.

The Trump administration insists that between those two vaccines and others in the pipeline, the U.S. will be able to accommodate any American who wants to be vaccinated by the end of the second quarter of 2021.

The decision not to lock in additional Pfizer purchases last summer was first reported by The New York Times. Health and Human Services Secretary Alex Azar told NBC the administration is "continuing to work across manufacturers to expand the availability of releasable, of FDA-approved vaccine as quickly as possible. … We do still have that option for an additional 500 million doses.”

The “Operation Warp Speed” summit will feature Trump, Vice President Mike Pence and a host of government experts, state leaders and business executives, as the White House looks to explain that the vaccine is safe and lay out the administration’s plans to bring it to the American people.

Officials from the pharmaceutical companies developing the vaccines were not expected to attend, despite receiving invitations, according to people familiar with the matter. Some expressed concerns about the event contributing to the politicization of the vaccine development process and potentially further inhibiting public confidence in the drugs.

Trump is set to kick off the event with remarks aiming to “celebrate" vaccine development, according to an official who previewed the event. Trump also will sign an executive order to prioritize Americans for coronavirus vaccines procured by the federal government. A second official said the order would restrict the U.S. government from donating doses to other nations until there is excess supply to meet domestic demand. Both officials spoke on condition of anonymity to discuss plans for the summit.

The Food and Drug Administration's panel of outside vaccine experts is to meet Thursday to conduct a final review of the Pfizer drug, and it will meet later this month on a vaccine developed by Moderna. The FDA is not required to follow the panel's advice, though it usually does.

Agency decisions on the two drugs are expected within days of each meeting. Both have been determined to be 95% effective against the virus that causes COVID-19. Plans call for distributing and then administering about 40 million doses of the two companies’ vaccines by the end of the year — with the first doses shipping within hours of FDA clearance.

Biden said Friday that “there’s no detailed plan that we’ve seen” for how to get the vaccines out of containers, into syringes and then into people’s arms.

Trump administration officials insist that such plans have been developed, with the bulk of the work falling to states and municipal governments to ensure their most vulnerable populations are vaccinated first. In all, about 50,000 vaccination sites are enrolled in the government's distribution system, the officials said.

Each of the forthcoming vaccines has unique logistical challenges related to distribution and administration. The Pfizer vaccine must be transported at super-cooled temperatures, and comes in batches of 975 doses. Each vial contains five doses, requiring careful planning. The administration has prepared detailed videos for providers on how to safely prepare and administer doses, to be posted after the FDA issues its emergency use authorization.

One such plan is to be announced Tuesday: Pharmacy chains CVS and Walgreens have stood up a “mobile vaccination service" ready to vaccinate people in every nursing home and long-term care facility in the country. The roughly 3 million residents of those facilities are among the most vulnerable for COVID-19 and have been placed at the front of the line to access the vaccine, along with more than 20 million healthcare workers. So far 80-85% of the facilities have signed on to the service, the officials said.

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AP writers Jonathan Lemire in Wilmington, Delaware, and Linda Johnson in Trenton, New Jersey, contributed to this report.