Jonathan Greene, deputy assistant secretary for operations and resources at the Department of Health and Human Services, acknowledged in a briefing call that the agency is not able to provide the number of health care workers states are requesting.

HHS and FEMA have been "very judicious" in using and moving health care workers, providing 300 to 400 nationwide, which matches the number deployed during the first surge in the spring, Greene said. Fewer were deployed over the summer, mimicking the case curve.

About 3,100 can be deployed nationally, but Greene admitted the system is designed more for situations like hurricanes than for pandemics.

“It works less well when we're talking about a pandemic where everyone needs all of these providers all at once. And certainly when we activate and deploy these people and take them out of their full-time jobs and put them somewhere else, it reduces the capacity in the places where they come from,” he said.

Greene said the agency has encouraged the use of non-traditional health care providers in order "to bridge the gap."

ABC News' Sarah Kolinovsky contributed to this report.

Ellen DeGeneres said Thursday that she’s tested positive for COVID-19 and is “feeling fine.”

“Anyone who has been in close contact with me has been notified, and I am following all proper guidelines,” the talk show host said.

New York City Mayor Bill de Blasio unveiled his 2021 student achievement plan, which will focus on getting students caught up after, for some, 18 months of remote learning.

“Clearly there will be a COVID achievement gap and we have to close that COVID achievement gap,” the mayor said.

Schools Chancellor Richard Carranza said the plan is to: get a baseline of what ground was lost; increase the high-quality digital curriculum available for every single school; launch a one-stop digital learning hub; deepen professional development; expand Parent University (the "online learning and empowerment platform" for families); and confront the trauma and mental health crisis within schools.

ABC News' Aaron Katersky contributed to this report.

In South Korea, more than 3,000 COVID-19 patients were identified in the last week, and in the Seoul area, as of Wednesday 506 were unable to be taken to hospitals this week due to bed shortages, Yonhap reported. 

South Korea reported 682 new cases and eight more deaths on Thursday, the Korea Disease Control and Prevention Agency reported.

The nation now has over 40,000 confirmed cases.

The first vaccines in the U.S. have now left the Pfizer facility and are on their way to all 50 states.

Two main trucks left on Sunday morning -- an additional truck will come in later to pick up a few boxes and take them to the U.S. territories.

On Monday Pfizer has an order for another 400 boxes of the vaccine as well as an order for a slightly less amount of boxes on Tuesday, the company said.

The first shipments are expected to arrive at 145 sites on Monday, 425 sites on Tuesday and another 66 sites on Wednesday, Gen. Gus Perna of the Trump administration's Operation Warp Speed vaccine program said.

At those sites, the vaccines will be paired with ancillary kits to assist with the vaccinations. The kits contain needles, syringes, alcohol swabs, diluent, safety gear and vaccine reminder cards.

-ABC News' Joshua Hoyos and Luis Martinez