Current and former U.S. health officials took to the airwaves Sunday to warn Americans of a potential jump in COVID-19 cases after the holidays.

“A surge upon a surge” may be on the way after the Christmas and New Year’s period, Anthony Fauci, the government’s top infectious-disease doctor, warned on CNN’s “State of the Union.”

Former U.S. Food and Drug Administration chief Scott Gottlieb said on CBS’ “Face the Nation” that “we have a grim month ahead of us” after a recent increase in cases, with hospitalizations rising on a lag of a few weeks.

The comments came as the number of confirmed cases in the U.S. passed 19 million and deaths at almost 333,000. Fauci, director of the National Institute of Allergy and Infectious Diseases warned that the U.S. is at a “very critical point” in combating the pandemic after many Americans ignored guidance to avoid travel.

Flying has picked up recently while remaining well below year-ago levels. Figures from the Transportation Security Administration show more than 1 million people moved through U.S. airport checkpoints on five of the past nine days heading into and through the holidays.

Adm. Brett Giroir, a member of the White House coronavirus task force, said the risks entailed by traveling depend mostly on what people do once they get to their destinations.

“What we really worry about is the mingling of different bubbles once you get to your destination,” he said on “Fox News Sunday.”

Health officials are also on alert for a more infectious COVID-19 strain that’s emerged in the U.K., although there’s no clear evidence it results in more severe cases of the disease.