(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.

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.

Before you go

Getting vaccinated is the key requirement for the vast majority of international travelers hoping to enter the United States.

Zwing lucked out with the US reopening date. When the travel ban hadn’t been lifted, she rebooked October flights for November 8 before the specific date was announced, hoping she’d finally be allowed to make the trip to the US.

“I couldn’t believe it at first but both me and my boyfriend are over the moon!,” she said when the date was finally announced in mid-October. “I can’t wait to hop on the plane on Nov 8th!”

For air travel

Unvaccinated Americans and a very limited number of unvaccinated international travelers exempted from the vaccination requirement must test within one day of departing for the US.

Many airlines have mobile apps and portals on their websites where vaccination and testing information can be processed digitally.

Hard copies are a good idea, too.

“Proof of who you are, your vaccination status, and front and back of a credit card can go a long way to turning the nightmare of losing your documents into a reasonable situation.”

At land borders

Customs and Border Protection anticipates an increase in travel volumes and wait times at land and ferry crossings and is encouraging travelers to have their identification and vaccination documents ready. The agency also encourages travelers to use its CBP One app.

Staffing levels will be at pre-Covid levels, according to CBP, but the agency will be balancing multiple priorities.

Digital and paper documentation is acceptable for proof of vaccination, and vaccine cards do not need to be in English.

Travelers should be prepared to attest to their vaccination status and reason for travel. They should also be prepared to show proof of being fully vaccinated, if requested by a CBP officer.

Children under 18 traveling with vaccinated adults are exempt from the vaccination requirement.

Covid tests are not required at land and ferry crossings.

The web of rules and requirements to travel internationally right now is undeniably tangled.

“Just hopping on a plane and going someplace doesn’t work anymore,” says travel adviser Dave Hershberger of Prestige Travel in Cincinnati, Ohio. “You’ve got to do your homework.”