The US Transportation Security Administration screened 1.28 million travellers at the country's airports yesterday, the highest number since the early weeks of the coronavirus pandemic in mid-March.
Health authorities had urged Americans to avoid travelling over Christmas, warning that crossing the country would increase their chances of catching and spreading the virus.
The Centres for Disease Control and Prevention recommended "postponing travel and staying home", as that was "the best way to protect yourself and others".
"You should consider making other plans, such as hosting a virtual gathering or delaying your travel," it said.
The TSA's figures are significantly lower than they were at the same time last year. Over the past 10 days, the agency has screened 10.2 million travellers, as opposed to almost 25 million a year ago.
But that is still the highest level of travel in the US since the start of the pandemic, exceeding the numbers seen over the Thanksgiving period last month.
It comes at a time when the US is suffering from uncontrolled spread of the virus.
The country is averaging 180,000 infections each day and 1400 deaths. Both numbers have come down off their peak a week ago, but as people tend to get tested less over holiday periods, that relief may not last.
Appearing on Sunday television, Dr Anthony Fauci said the worst period of the pandemic was likely still ahead of the US, due in no small part to the amount of travel happening over the holidays.
"We very well might see a post-seasonal – in the sense of Christmas, New Year's – surge. And as I've described it, a surge upon a surge," Fauci told CNN.
"Because if you look at the slope, the incline of cases that we've experienced as we've gone into the late fall and soon to be early winter, it is really quite troubling.
"We really are at a critical point."
The US is also firmly in its winter months now, meaning people will be congregating inside where the virus spreads more easily.
Fauci has predicted the general population in the US will be getting immunised widely by late March or early April — beyond the front-line workers, older people and certain other segments of the public given priority for the vaccines.
- additional reporting: AP