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.

U.S. Surgeon General Jerome Adams urged people to take the coronavirus pandemic seriously and follow public health guidelines as infections and hospitalizations soar across the country.

"I want the American people to know this virus is incredibly unforgiving, cases are going up, hospitalizations are going up, my colleagues are dog-tired and we need you to hang on just a little bit longer because we've got vaccines coming but we want as many people to be alive to get them as possible and a lot of that is going to depend on your behavior," Adams told ABC News chief anchor George Stephanopoulos in an interview Monday on "Good Morning America."

Although "more people than ever are wearing masks," Adams said he's "very" concerned by the number of people who still "don't understand how much spread is occurring by people who don't have symptoms."

About 20 million "full doses" of COVID-19 vaccines will be available by the end of the year, according to Adams, who urged every American to be immunized against the virus as soon as a vaccine is authorized and made available.

"It's a way that we can ultimately end this pandemic, but it doesn't matter if people won't get the vaccination," the surgeon general said. "We know that vaccine levels are only about 50% for adults for flu and they go down to about 40% for African-Americans."

Adams said he is working with historically black colleges and universities as well as faith communities to bolster vaccine confidence among all populations. The White House is also hosting a vaccine summit Tuesday, he said.

"One thing you can all do right now, it's national flu immunization week, get your flu shot because half a million people were hospitalized last year with the flu," Adams said. "We simply can't afford for that to happen this year with hospitals being overwhelmed."

The surgeon general said the current surge in infections across the country "is different than earlier surges," because it's not about a lack of masks or personal protective equipment, nor is it due to a lack of testing.

"It's really about health care capacity, and certain places are just being overwhelmed," he said. "So we know that we can actually help them with their health care capacity by immunizing their health care staff. We're going to leave it up to the states, but we're going to give them guidance."

Adams noted it's also important to vaccinate those who are most likely to die from COVID-19.

"We know that 40 to 50% of the deaths are occurring in people who are in longterm care facilities who are older," he said.