Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told ABC's "This Week" that the level of infection in the USA would not "all of a sudden turn around".
"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.
"What we expect, unfortunately, as we go for the next couple of weeks into December, is that we might see a surge superimposed on the surge we are already in", he said.
The trend is ominous, Fauci and other government scientists said, with the Christmas holidays soon bringing more travel and family gatherings.
As of Sunday, the number of Covid-19 cases recorded in November have surpassed four million, double the figure recorded in October.
"We don't want to frighten people, but that's just the reality" Fauci added.
Deborah Birx, White House coronavirus response coordinator, noted a surge in Covid-19 after a holiday weekend in May. "But as I said in the past, the default position should be to try as best as possible within reason to keep the children in school, or to get them back to school".
"I can't see how we're not going to have the same thing", he explained.
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. "The part at the distal end, namely, getting it into people's arms, is going to be more challenging than a regular flu season, it would be foolish to deny that".
Ethiopian military has taken 'full control' of Tigray capital, government says
Earlier, TPLF leader Debretsion Gebremichael told Reuters in a text message that Mekelle was under "heavy bombardment". At least one rocket fired from Tigray targeted Eritrea's capital Asmara on Friday night, regional diplomats said.
Fauci's warning came as more than 90,000 COVID-19 patients are now hospitalized across the United States, with over 18,000 in intensive care units, according to the COVID Tracking Project.
Pfizer was using charter flights to pre-position vaccines for quick distribution once it receives USA emergency authorization - expected as early as December 10 - the Wall Street Journal and other media reported. "But the American people have to do the right things until we get that vaccine widely distributed".
America's top infectious disease expert said on Sunday that the United States may see "surge upon a surge" of the coronavirus in the weeks after Thanksgiving, and he does not expect current recommendations around social distancing to be relaxed before Christmas.
"About 20 per cent of all people in the hospital have Covid, so this is a really unsafe time", Mr Giroir said.
It means measures to control the spread of the virus have been left to state and local leaders, with many imploring people not to travel this Thanksgiving - with limited success.
"So hang on just a little bit longer".
Europe on Saturday crossed a grim barrier, registering 400,649 deaths.
Germany, once a beacon of hope in Europe's coronavirus nightmare, reached on Friday the mark of more than one million cases.