The new requirement announced this week is part of a strategy that includes the extension of a mandate that people on commercial aircraft and in other transportation settings wear masks, as well as the expansion of a program offering voluntary testing for new arrivals at key U.S. airports.
Airline and travel industry officials said Friday they were generally supportive of the temporary measures, seen as a way to ensure passenger health and confidence in flying. The new rules call on airlines to build upon a foundation for testing created earlier in the pandemic.
During his announcement of the changes, President Joe Biden said the tighter testing timeline would provide an added degree of protection for travelers as scientists continue to study the variant. The new measures come on the heels of a decision last week to ban travel from eight nations in southern Africa.
At least 12 cases of the omicron variant, first detected in southern Africa last month, have been discovered in the United States, including three on Friday in Maryland.
Henry Wu, associate professor of infectious diseases at the Emory University School of Medicine and director of the Emory TravelWell Center, said the new testing requirement could be effective in catching infections that previously might have gone undetected.
"What they're doing is making the holes in the net a little smaller," he said. "They're going to detect more cases that would have been on the plane and keep [those individuals] from traveling."
Carter Yang, spokesman for the airline trade group Airlines for America, said Friday the group "strongly supports" the administration's testing and contact-tracing requirements, as well as vaccination verification measures.
"U.S. airlines are fully prepared to implement the new requirement that passengers on inbound international flights submit proof of a negative test no more than one day prior to departure," he said.
Other industry groups said they are supportive but cautioned against making the requirements - particularly for testing - permanent.
"We hope this measure to narrow the pre-departure testing window will be temporary until more is learned about the Omicron variant," Tori Barnes, executive vice president of public affairs and policy for the U.S. Travel Association, said in a statement.
The impact of the new variant on travel demand is not clear. Air travel approached pre-pandemic levels during the Thanksgiving holiday, when the Transportation Security Administration screened more than 2.4 million people the Sunday after Thanksgiving - its busiest day since the start of the pandemic.
George Ferguson, a senior aerospace analyst, and Francois Duflot, a senior associate analyst, both with Bloomberg Intelligence, wrote in a report released Friday that the variant threatens to dampen demand for air travel in early 2022 and possibly through the spring travel season. The virus also could prompt carriers to cut back on international offerings, according to the report.
"Airline prospects are deteriorating as rising delta variant cases combine with risks that vaccines are less effective against the new omicron strain," the report said.
Two U.S. carriers, Delta Air Lines and United Airlines, which operate flights between the United States and South Africa, said they had no plans to reduce service. United chief executive Scott Kirby told the Financial Times this week that the variant might have a "short-term impact" but that the "long-term forecast is not any different than it was before."
Under rules outlined in October as the United States prepared to lift its ban on international travel from 33 countries last month, travelers who had been vaccinated could take a virus test within three days of their departure. Others were required to be tested within one day.
Airlines, which are charged with verifying testing results, say they are ready to implement the new requirements. Many have had systems in place for months to validate test results and vaccine statuses. Several carriers also have formed partnerships with organizations that provide customers with access to testing centers.
United's partnership with TrustAssure allows customers to find testing options that meet U.S. entry requirements. Customers can then access that information through United's website or app.
American Airlines' Ready to Fly checklist enables customers to submit testing results and vaccination records, as well as personal information for contact-tracing purposes. Those who have been cleared can check in online or via the airline's app.
Wu said the narrower testing window for travel to the United States could pose challenges in some parts of the world, where access to fast, reliable testing is not the norm. The administration is trying to address those concerns, officials said.
At a briefing Friday at the White House, Jeffrey Zients, the administration's coronavirus response coordinator, said the administration will continue to be flexible with its strategy for combating the virus but said there is no timeline for when measures such as travel restrictions will be relaxed.
"As we learn more about the variant over the coming days, on all of these fronts, decisions on changes to travel policy will be based on the advice, as they always are, of our health and medical experts," he said.