The U.S. allows vaccinated international travelers in and bars unvaccinated travelers from abroad.

The United States reopened its borders for fully vaccinated travelers from dozens of countries on Monday, ending more than 18 months of restrictions on international travel that left families separated from loved ones and cost the global travel industry hundreds of billions of dollars in tourism revenue.

Under the new rules, fully vaccinated travelers will be allowed to enter the U.S. if they can show proof of vaccination and a negative coronavirus test taken within three calendar days of travel. Unvaccinated Americans and children under the age of 18 are exempt from the requirement, but must take a test within one day of travel.

The shift has come in time for the holiday season, when the beleaguered tourism industry is eagerly awaiting an influx in international visitors, especially in popular big-city destinations like New York, Los Angeles and Miami. The extended ban on travel from 33 countries — including European Union members, China, India and Iran — devastated the sector and resulted in losses of nearly $300 billion in visitor spending and more than one million American jobs, according to the U.S. Travel Association.

“Monday begins in earnest the return of international travel, when long-separated families and friends can safely reunite, travelers can explore this amazing country, and the U.S. is able to reconnect with the global community,” said Roger Dow, the Association’s president and chief executive officer. “It is a monumental day for travelers, for the communities and businesses that rely on international visitation, and for the U.S. economy overall.”

Airlines saw a big spike in online searches and ticket bookings for international travel — particularly from Europe and Latin America — after the administration announced its plans to ease the travel restrictions in September.

American Airlines said bookings over the three days after the announcement were up 66 percent for flights between Britain and the United States, 40 percent for those from Europe and 74 percent for Brazil, compared to a similar period a week earlier. United Airlines said that it sold more tickets for trans-Atlantic flights in the days after the announcement than during a similar period in 2019, a first since the pandemic began.

Delta Air Lines said that many of its international flights on Monday were fully booked. The carrier’s first flight into the United States under the looser restrictions, DL106, is expected to arrive from São Paulo, Brazil, at 9:35 a.m. on Monday, in Atlanta. By the end of Monday, Delta expects to fly 139 mostly full planes from 38 countries into the U.S.

Hotels across the country, particularly those in cities, also felt the impact of the reopening announcement, with increased bookings and interest over the holiday season. Hyatt, the hotel group, said that approximately 50 percent of its bookings by international travelers to the U.S. for the week of Nov. 8 came after the date was announced in mid-October, with travelers flocking to top cities.

The chef Daniel Boulud, who owns several restaurants in New York City, said that customers from overseas had already started to call for reservations or to go on a waiting list.

He added that while his restaurants were already “quite busy,” buoyed by domestic tourism and a trickle of international visitors, “the faucet was not open for tourism yet.” International tourists, he said, will bring necessary foot traffic, in particular to his restaurants near the Theater District.

Many of the airplanes departing for the United States over the coming weeks will be full of travelers reuniting with family and friends after more than a year apart. Felicity Fowler, a retired homeopath from London, missed the birth of her grandson. He was born in New York in April; she hasn’t seen her daughter, his mother, since February 2020 .

“It’s been emotional torture to be so far away from my girl at a time when she has needed me the most,” she said in an interview. “We need to make up for lost time.”

While demand is up, the changes have not had a major immediate effect on the number of scheduled international flights. In the first week under the relaxed rules, airlines will operate an average 1,390 daily flights into the United States, up just 35 daily flights from the week before, according to figures from Cirium, an aviation data provider. The data does not, however, indicate how many seats have been sold per flight and the number of daily flights is expected to rise again, by 40, in the second week after reopening.

With the continued risks of coronavirus variants and uncertainty of the course of the pandemic, the U.S. Travel Association does not expect international inbound travel to recover to 2019 levels until at least 2024.