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. approaches 19 million and deaths are more than 332,000. 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 levels from a year ago. 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, because being on an airplane is typically safe.

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

Vaccinations in the U.S. began Dec. 14 with health care workers and residents of nursing homes. So far almost 2 million doses have been administered in the country, according to a state-by-state tally compiled by Bloomberg. Those numbers are accelerating as a second vaccine by Moderna Inc. is distributed.

Mutant strain

Health officials are on alert for a more infectious COVID-19 strain that's emerged in the United Kingdom, Germany, Switzerland, Ireland and Japan, although there's no clear evidence it results in more severe cases of the disease.

Travelers coming to the U.S. from the U.K. face more restrictions because of the new variant, with the U.S. insisting on testing negative for COVID-19 within 72 hours of departing the country.

Gottlieb, a board member of Pfizer Inc., said that he believes the U.K. strain is already in the U.S., and in "a reasonable number at this point."

He saw signs that U.S. new daily COVID-19 cases are starting to plateau, but since hospitalizations and the number of deaths tend to lag by two to three weeks, "we have a very difficult month ahead of us."

President Donald Trump has remained silent about the new variant and the worsening crisis gripping some parts of the U.S. He took to Twitter on Saturday to criticize Democratic governors for taking emergency steps to contain the outbreak.

"The lockdowns in Democrat run states are absolutely ruining the lives of so many people," Trump tweeted.