Everyone who traveled over Thanksgiving to meet with family should assume they have COVID-19 and get tested in the coming week, Dr. Deborah Birx, the coordinator of the White House Coronavirus Task Force, told CBS's "Face the Nation" on Sunday.
She also appeared to suggest that families who split up for Thanksgiving should wear masks in their home.
Experts are "deeply concerned" about the impact of Thanksgiving travel on COVID-19 rates in the US, she warned. It could cause a spike in cases, she said.
"If you're young and you gathered, you need to be tested about five to ten days later," she said. "But you need to assume that you're infected and not go near your grandparents and aunts and others without a mask.
Families should also wear masks indoors "if they chose to gather during Thanksgiving and others went across the country or even into the next state."
She later added, without the reference to young people: "If your family traveled, you have to assume that you were exposed and you became infected and you really need to get tested in the next week."
Many people met relatives over Thanksgiving, including some who traveled to different states, Birx said, adding that "people may have made mistakes."
The Centers for Disease Control and Prevention had warned against holiday travel, but around a third of Americans said this didn't affect their plans, according to a Business Insider survey.
The day before Thanksgiving was the busiest air travel day since March 16, and also the day where the US recorded its highest single-day coronavirus death toll since early May.
After Memorial Day in May, cases in the US surged, Birx said — but this was over a lower baseline than right now, with less than 25,000 new cases per day.
The daily case rate is now more than seven times higher than that, at around 180,000, she said.
She echoed the view of Dr. Anthony Fauci, the top infectious disease expert in the US, that the post-Thanksgiving rise in cases could cause a peak within the current wave, or even a fourth surge.
"What we expect, unfortunately, as we go for the next couple of weeks into December: that we might see a surge superimposed upon that surge we're already in," Fauci had told NBC's "Meet the Press" earlier on Sunday.
Birx also urged Americans to take personal responsibility for reducing the spread of the virus, even if their state doesn't have a mask mandate or a rule to limit gatherings in public spaces such as restaurants.
"To every American, this is the moment to protect yourself and your family," she said.
Birx praised the impacts of mask mandates. They can make a "significant difference" to hospitalization levels and the number of fatalities, she said.
Alongside these mandates, states and counties should close or reduce the capacity of spaces where people cannot wear masks, such as bars and indoor restaurants, she added.
Arizona previously got control of the virus from closing bars and restaurants, she said, and the state should consider closing them again amid the current rise in cases.
Birx's team hopes to brief the Biden administration on Monday, she added. This will primarily involve discussing how best to share data.
She said she is briefing Vice-President Mike Pence, chair of the task force, every day, but didn't respond to a question about when she last met President Donald Trump.