The United States is opening to vaccinated international travelers on Monday.

(CNN) — The United States is flinging its doors wide open to vaccinated international travelers on Monday, welcoming many visitors who’ve been shut out of the country for 20 months.

With new requirements going into effect for 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 are 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.

John F. Kennedy International Airport in New York (pictured in January 2021) will welcome more international flights on Monday.

Spencer Platt/Getty Images

International flight arrivals will be up 11% on Monday over a similar day in October at Newark Liberty and John F. Kennedy international airports, with 253 flights scheduled to arrive, according to the Port Authority of NY & NJ.

The Port Authority expects a gradual increase at those airports over the next two months, with international traffic reaching about 75% of November and December 2019 levels, based on current international schedules which are likely to change.

Customs and Border Protection (CBP) and the Transportation Security Administration have also been short staffed and face a looming deadline right before Thanksgiving for federally mandated employee vaccinations.

Increased wait times are expected by CBP at busy land border crossings from Canada and Mexico.

All this to say, preparation and patience will be key for international travelers heading into the US.

Here’s what travelers can expect and prepare for:

When you get there

The United States is largely wide open, although there are some state and local restrictions that still apply.

For example, there are mask mandates in Hawaii, Illinois, New Mexico, Nevada, Oregon and Washington. Washington, DC and Puerto Rico also require masks in indoor public spaces.

In some cities, including New York and San Francisco, there are vaccine requirements for indoor public spaces including restaurants.

Hawaii, which had some of the strictest entry requirements in the US, will now align with the new federal rules for international air travel. Although capacity restrictions in the state are easing, there are still some limits in place.

California (with Santa Monica pictured here), New York and Florida are leading destinations for incoming international travelers.

Daniel Slim/AFP via…