Airlines were seeing a travel rebound before Omicron arrived.

For months, airline travel has been steadily rebounding, and Sunday was the busiest travel day at U.S. airports since February 2020. But the discovery of the Omicron coronavirus variant threatens to derail the industry’s recovery, as the Delta variant did this summer.

Several nations, including the United States, have barred visitors from South Africa and a handful of neighboring countries. Japan, Morocco and Israel have barred all incoming foreign visitors, while the Philippines has banned visitors from southern Africa and several European countries.

The tightening of restrictions has drawn criticism from the travel sector. In a statement last week, Willie Walsh, the head of the International Air Transport Association, a global trade association, called for “safe alternatives to border closures and quarantine.” Over the weekend, the U.S. Travel Association urged the Biden administration to rethink its ban.

“Covid variants are of concern, but closed borders have not prevented their presence in the United States while vaccinations have proven incredibly durable,” Tori Emerson Barnes, executive vice president for public affairs and policy, said in a statement. “With a vaccine and testing requirement in place to enter the U.S., we continue to believe that assessing an individual’s risk and health status is the best way to welcome qualified global travelers into the United States.”

For U.S. airlines, the rebound in international travel has been slower than that for travel within the United States. But President Biden’s decision to ease longstanding restrictions on foreign travelers this month promised to stimulate that recovery. It isn’t yet clear whether or how the Omicron variant will affect travel demand, but if travel bans proliferate and concerns over the variant continue to spread, hopes for an accelerated international rebound could be dashed again.

Only two U.S. carriers, Delta Air Lines and United Airlines, fly out of southern Africa. Both have said that they are not yet planning to adjust their schedules in response to the administration’s ban, which took effect on Monday and does not apply to American citizens or lawful permanent residents. Delta operates three weekly flights between Atlanta and Johannesburg. United operates five flights a week between Newark and Johannesburg, and it has not changed its plans to restart flights between Newark and Cape Town on Wednesday.

No major American airline has announced any substantive changes to procedures because of the variant. And all passengers flying into the United States must provide proof of a negative coronavirus test, with noncitizens also required to be fully vaccinated.

Within the United States, air travel has nearly recovered, even with many businesses still wary of sending employees on work trips. The number of people screened at airport security checkpoints over the past week was down only 12 percent from the same week in 2019, according to the Transportation Security Administration.

The industry easily handled the crush of travelers over the holiday week, avoiding the disruptions that lasted for days at some airlines in recent months. In the seven days ending Sunday, there were fewer than 600 cancellations, accounting for less than 0.5 percent of all scheduled domestic flights, according to FlightAware, an aviation data provider.