Millions of Americans curtailed their traditional family gatherings for last Thursday's annual Thanksgiving holiday, yet millions of others ignored warnings from health care experts against traveling to visit far-flung relatives for fear of spreading the virus.
Twenty-six states have set records for COVID-19-linked hospitalizations this past week, CNN reported.
Dr Fauci also appeared on NBC's "Meet the Press", where he made similar remarks, adding that it's "not too late" for people travelling back home after Thanksgiving to help stop the spread of the virus by wearing masks, staying distant from others and avoiding large groups of people.
First it was Thanksgiving, now it's Christmas. "Now we are deeply anxious about what could happen post-Thanksgiving because the number of cases, 25,000 versus 180,000 a day, that's where - that's why we are deeply concerned", Birx said, before adding that there was some improvement in the Northern Plains states prior to the holiday.
The White House's COVID testing czar also told CNN that people should get tested if they traveled over the holidays, which he said is "really concerning to all of us".
Fauci told MSNBC that the task force expects a "surge superimposed upon that surge that we're already in" in the coming weeks. But I think it's going to be able to get done because the local people have done that in the past.
The number of new COVID-19 cases reported in the United States topped 200,000 for the first time on Friday.
Interior Health monitoring community COVID-19 cluster in Revelstoke
Of the new cases confirmed on Friday, more than half involved residents in either Toronto (494 new cases) or Peel (517). According to the province's epidemiology report, 13 of the 20 deaths were people living in long-term care homes.
The company is using charter flights to pre-position vaccine for quick distribution once it receives U.S. approval, which is expected as early as December 10, the Wall Street Journal reported.
However, Fauci did note that there is "light at the end of the tunnel", explaining that the coronavirus vaccines will begin to be distributed to a portion of Americans by the end of December.
And New York City again took a small step back toward normality, as Mayor Bill de Blasio announced that elementary schools would reopen for in-person instruction on December 7.
"We likely will have an increase in cases as we get into the colder weeks of winter and as we approach the Christmas season", he added. Pfizer, whose vaccine is reportedly up to 95% effective, says it can have tens of millions of vaccine doses ready by December.
But like Dr Fauci and the other scientists, he expressed grave concerns about the months immediately ahead.
"About 20 percent of all people in the hospital have Covid, so this is a really unsafe time", Giroir said.
"We likely, nearly certainly, are going to be vaccinating a portion of the individuals in the first priority [healthcare workers] before the end of December, and then as we get into January and February and March, more and more", Dr Fauci said.
It can sometimes take two weeks for infected people to develop symptoms, and asymptomatic people can spread the virus without knowing they have it.