The United States flung its doors open to vaccinated international travelers on Monday, welcoming many visitors who’ve been shut out of the country for nearly 20 months.

For Maria Giribet Caldentey, it marked a precious opportunity to see her twin grandchildren in California. She’s already had to postpone this trip once as the border was closed.

“I’m leaving the first day they will allow me to leave… I’ve got butterflies. I had a few anxieties but I’m delighted to think that I’m going to be able to go see them,” she told CNN at Paris’ Charles de Gaulle airport.

It almost felt like business as usual there on Monday morning, with a steady stream of masked passengers flowing through the terminal.

But with new requirements going into effect for US air, land and ferry arrivals, there’s bound to be some congestion as the rules are rolled out.

“It’s going to be a bit sloppy at first, I can assure you,” Delta Air Lines CEO Ed Bastian said at a late October travel industry conference. “There will be lines, unfortunately,” he said, citing “an onslaught of travel all at once.”

Many Delta flights due to arrive on Monday were 100% full with high load factors in the following weeks, according to Delta spokesman Morgan Durrant. Delta has seen a 450% increase in international bookings in the six weeks since the US reopening was announced, he said.

The Lufthansa Group told CNN that its carriers were operating 31 flights to the US on Monday, all fully booked. Virgin Atlantic said its flights were 98% full while both Swiss Air and British Airways said passenger volume on flights departing to the US was high.

Source: Civil Aviation Authority-Qatar