Top US diseases expert Dr Anthony Fauci has warned of another surge in Covid cases after Christmas - even with the rise following Thanksgiving still being tackled.
He said the longer Christmas/New Year period may be even more of a challenge.
The US is seeing peak infections of close to 200,000 a day on average with record numbers of people in hospital.
California is under a strict new lockdown, with other states announcing record increases.
The US has recorded more than 14.7 million cases of infection in the pandemic so far and 282,500 deaths, according to Johns Hopkins University research, both global highs.
President Donald Trump has been accused of playing down measures such as mask wearing but the US has also had to deal with different states taking different approaches to tackling the virus.
What are Fauci and other experts saying?
Dr Fauci, who has been asked by President-elect Joe Biden to be his Covid chief medical adviser, told CNN his concerns for Christmas were the same as his concerns for Thanksgiving, "only this may be even more compounded because it's a longer holiday".
He said nobody wanted to modify or shut down the holiday season, but "we're at a very critical time... we've got to not walk away from the facts and the data. This is tough going for all of us".
Millions defied appeals from experts not to travel over the Thanksgiving period - the Sunday after the Thursday 26 November holiday saw the most air travellers since March and the Covid effects are still to be fully felt.
Dr Fauci, who has been part of the Trump administration's response to Covid-19 but has also drawn criticism from the president for his views, said people tended to return to work the week following Thanksgiving but that this is often not the case with Christmas leading into New Year.On Sunday, former Food and Drug Administration (FDA) commissioner Scott Gottlieb said US deaths could be near 400,000 by the end of January, adding: "As bad as things are right now, they're going to get a lot worse."
Also on Sunday, Dr Deborah Birx, the White House coronavirus task force co-ordinator, criticised the Trump administration for flouting guidelines and peddling "myths" about the pandemic.