• Despite omicron, airlines are expecting some of their busiest days of the Covid pandemic during the year-end holidays.
• New travel rules since omicron was detected late last month have complicated international trips.
• Many travelers need Covid tests in order to fly to certain countries, including the U.S.

Travelers are scrambling for Covid tests ahead of holiday trips, facing long lines and longer waits for results.

Lines for coronavirus testing at some walk-in clinics in New York wrapped around the block on Tuesday. Pharmacies ran out of at-home tests. Walgreens limited purchases of at-home tests to four per customer. CVS Health capped them at six per person.

XpresCheck, which offers testing at several major U.S. airports, said it's not running out of tests but it has hit appointment capacity during certain periods of the day, Doug Satzman, CEO of parent company XpresSpa, said by email.

The White House on Tuesday said the government will buy 500 million Covid tests that people can order for free online, starting in January. But the testing crunch is already upending some holiday plans.

Britt Groosman, an economist and program manager at the Environmental Defense Fund, canceled her family's Christmas vacation to see their relatives in Belgium and the U.K. because of the rampant omicron variant and difficultly getting test results. It would have been their first time spending the holiday there in two years, she said.

Because of omircron's spread, New York City-based Groosman said that she "would not have felt comfortable that a test taken 48 hours before [departure] would have been sufficient to feel comfortable visiting elderly parents."

Groosman took a PCR test for herself on Saturday and by Tuesday still hadn't received the results. An American Airlines representative told her the carrier would give her a credit for the roughly $4,000 she paid for four tickets, she said.

Omicron has quickly become the dominant variant of sequenced Covid cases. Despite the spread, most travelers are sticking to their holiday flights.

The Transportation Security Administration on Tuesday screened 1.98 million people, snapping five days of daily screenings above 2 million, the longest stretch since Thanksgiving week. United and Delta are expecting their busiest days since the beginning of the pandemic during the year-end holiday period — though still shy of 2019 levels.

Delta said it expects to fly 7.8 million passengers from Dec. 17 through Jan. 3, about 16% less than before the pandemic but the busiest days of the crisis so far.

United last week said it expects to carry 420,000 passengers a day between Dec. 16 through Jan. 3, up from 400,000 during Thanksgiving, but down about 13% from two years ago.

United CEO Scott Kirby said international travel will likely be the most impacted. Travel restrictions have changed rapidly since the omicron variant was first detected late last month, including in the U.S., where vaccinated citizens and visitors alike, starting this month, need to show proof of a negative Covid test taken within a day of departure for the U.S., up from three days ahead.

"Here domestically in the United States, demand has not dropped off," Kirby said in an interview with ABC's "Good Morning America" on Monday. "Internationally, it's dropped off pretty significantly as borders have closed and there's lockdowns overseas. It's really the tale of two different cities."

— CNBC's Spencer Kimball contributed to this report.