That decision could delay the delivery of a second batch of doses until manufacturer Pfizer fulfills other international contracts.
The revelation, confirmed Monday by people familiar with the matter, came on the eve of 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.