Stricter coronavirus testing rules take effect for travelers flying to the U.S.

The United States began requiring international travelers on Monday to provide proof of a negative coronavirus test taken no more than a day before they set out. The move, intended to limit the spread of the Omicron variant, is causing headaches for many passengers.

Previously, fully vaccinated travelers could provide proof of a negative test taken within 72 hours of departure. The new requirement may be difficult for some to satisfy, because it can take more than a day to receive test results.

The new rules have some travelers wondering if they can stick to their planned itineraries. They are one more hurdle to clear for Americans who are living outside the United States and for foreigners hoping to visit for the Christmas and New Year’s holidays. From London to Taipei, travelers have been thinking about the scenarios that could emerge on a trip, like what would happen if a flight is canceled or if the traveler tests positive along the way.

August Dichter, 24, said on Monday that he had already spent two to three hours trying to figure out how to meet the testing requirement for his scheduled flight on Thursday to Philadelphia from London. Mr. Dichter, an American who just completed a yearlong master’s degree program in Wales, said he had gotten conflicting messages from the airline, with some guidelines describing the new requirement and others still saying he had a 72-hour window.

Mr. Dichter said he had been looking forward to traveling around Europe during his studies, but that it had not been easy.

“It’s been a lot of hoops to jump through, and I know that I’m going to be able to jump through them all,” he said. “But they seem to just keep being so tedious, and to add up, and make the arrival of coming home feel just a little further away.”

Sharon Harkishin, 44, who plans to travel to Guyana from London later this month with a layover in New York, said she had stopped at three pharmacies to compare the prices of P.C.R. tests that would provide results within 24 hours. “It’s so expensive,” she said at a Boots pharmacy in North London.

Suj Shah, the owner of Clan Pharmacy in North London, said a number of American customers had come in who were unsure whether the tests could be done any time on the day before travel, or had to be done within 24 hours of the flight. (Mr. Shah said anytime the previous day was fine.)

“They’re not aware of the updates until we tell them,” Mr. Shah said of the customers.

More than a dozen countries around the world, including the United States, have gone a step beyond testing requirements and have barred travelers who have recently been in any of eight southern African countries. Health experts have criticized that policy and have urged caution, because so little is known yet about the Omicron variant, which was first detected and sequenced less than two weeks ago in South Africa.

Dr. Rochelle P. Walensky, the director of the Centers for Disease Control and Prevention, tried to add some perspective on Sunday on the ABC News program “This Week.”

“What we don’t yet know is how transmissible it will be, how well our vaccines will work, whether it will lead to more severe disease,” Dr. Walensky said.

The United States’s stricter testing requirement for inbound travelers took effect just as airline travel was experiencing something of a rebound. The Sunday after Thanksgiving was the busiest travel day at U.S. airports since February 2020, according to the Transportation Security Administration.