Despite CDC warnings, millions of people flew over the holidays

Even with the uptick in travelers over the holidays, the Transportation Security Administration (TSA) revealed Monday that it screened 500 million fewer passengers in 2020 compared to last year -- a 60% drop.

"Between Jan. 1 and Dec. 31, 2020, the agency screened a total of approximately 324 million passengers throughout its airport security checkpoints," TSA said in a news release. "That figure represents just 39 percent of the approximately 824 million total passengers screened in 2019."

Despite warnings from the Centers for Disease Control and Prevention advising people not to travel, TSA screened almost 18 million of those people over the holiday travel period. The last day of the travel period, Jan. 3, marked the highest checkpoint volume since the pandemic hit, with TSA screening 1,327,289 people.

The total number of holiday fliers was still down around 40% compared to last year, but it greatly exceeded predictions. AAA had forecasted only 2.94 million would travel by air between Dec. 23 and Jan. 3.

Top infectious disease expert Dr. Anthony Fauci was worried that Christmas could be worse than Thanksgiving in terms of potential COVID-19 spread because Christmas is a longer holiday. After each summer holiday, the U.S. reported a significant rise in infections across the country, and experts say Thanksgiving has played a major role in the country's largest viral surge to date.

On ABC's "This Week," Fauci said cold weather forcing people indoors paired with "the traveling associated with the holiday season is all of the ingredients that unfortunately make for a situation that is really terrible."

"To have 300,000 cases in a given day and between 2,000 and 3,000 deaths per day is just terrible," he said Sunday. "It's something that we absolutely have got to grasp and get our arms around and turn that -- turn that inflection down by very intensive adherence to the public health measures uniformly throughout the country with no exceptions."

Although the International Air Transport Association (IATA) estimates that air travel won't return to pre-pandemic levels until 2024, the record-breaking development of the Pfizer and Moderna vaccines has given some airline executives hope that demand will return sooner.

On Friday, Delta Air Lines CEO Ed Bastian told employees in an internal memo that Delta expects to achieve positive cash flow by the spring.

"The second phase will begin only when we reach a turning point with widely available vaccinations that spur a significant return to travel," Bastian wrote, "particularly business travel."

TSA said on Monday it "anticipates daily travel volumes will continue to rise steadily and follow seasonal patterns" but it "expects volume will remain well below pre-pandemic levels through most of 2021."

ABC News' Arielle Mitropoulos and Sam Sweeney contributed to this report.