Coronavirus updates: US reports nearly 300,000 new cases in all-time high

The Transportation Security Administration (TSA) on Sunday saw the highest number of people screened at the airport since the pandemic hit, a spokesperson tweeted Monday.

The 1,284,599 people screened at airports nationwide marks the sixth day in the last 10 with more than 1 million airline screenings. "If you choose to travel, please wear a mask," Lisa Farbstein, the TSA spokesperson, wrote on Twitter.

-ABC News' Michael Kreisel contributed to this report.

Health experts have been worrying about a third wave of COVID-19 hitting the U.S. after the holidays, as already 9 million people have traveled during the season. But speaking to "Good Morning America" Monday morning, Adm. Brett Giroir, the assistant secretary for health at the U.S. Department of Health and Human Services (HHS), said the country may not see another big spike in cases.

"How bad it will get really depends on what people do. After Thanksgiving, in the Midwest and the Northern Plains, we did not see a spike in cases, and in fact, it continued to go down," he said.

He added that while traveling does put people at higher risk of contracting the disease, we will not necessarily see another spike if people follow the rules.

"Limit travel if you can. If you're sick, please don't travel. Always wear a mask and watch your distance. And be careful, it's not really the travel, but it's mixing your bubble with a new bubble once you get there," Giroir said.

He added that this week, 4.7 million doses of a COVID-19 vaccine will be delivered to the U.S., getting the country closer to the government's goal of vaccinating 20 million Americans by the end of the year.

Adm. Brett Giroir, the assistant secretary for health at the U.S. Department of Health and Human Services (HHS) spoke to "Good Morning America" Monday morning to discuss the new coronavirus variant taking over the U.K.

"We don't have proof that it's here, but we do suspect that it is likely here, given the global interconnectedness," Giroir said. "We have no evidence that it's here. It's certainly not widespread here, but we need to look and make sure it's not here."

He added that while "there is increasing evidence that it really is more transmissible" or contagious, due to the viral load that people with that strain have been shown to have, there is "no evidence that it is more serious."

There is no evidence that people who become infected with the variant are more likely to be hospitalized or die, Giroir said.

"And we still believe -- don't have absolute proof -- but we have very good evidence and a good belief that the vaccines will still be effective," he added.

Huntington Hospital in Pasadena, California, released a patient information sheet documenting how they will use their medical resources should they see a shortage due to an overwhelming number of new patients following the holidays.

"We are not currently in this situation, but could be based on ongoing increase in hospitalized COVID-19 patients," they told ABC News, adding that on Sunday they had their highest number of patients (189) in a single day.

In the letter, the hospital explained that due to rising cases of COVID-19 in the community, they may experience limited resources. These include life support machines (like a ventilator or breathing machine), intensive care unit (ICU) beds and healthy medical staff to care for patients.

If there is a shortage of resources, a team of medical professionals will review the cases of all patients who are critically ill to determine how these resources should be shared throughout the hospital. 

"If a patient becomes extremely sick and very unlikely to survive his/her illness (even with life-saving treatment) – limited medical resources may go to treat other patients who are more likely to survive," the letter reads.

"Our community is facing a public health emergency that has severely constricted the medical resources available to patients in the Los Angeles County and greater Southern California region. Hospitals such as ours are working hard to meet the dramatic rise in needed care during this COVID-19 surge. We expect to face additional challenges moving forward after the holiday season," the hospital said in a statement.

ABC News' Abigail Shalawylo contributed to this report.