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.

Anna Zwing, 28, is flying from Germany to Chicago on Monday to see her boyfriend for the first time in more than two years. She checked in for her Lufthansa flight in person in Frankfurt on Sunday to make sure her ESTA application and all the new Covid documentation she has vigilantly gathered and double- and triple-checked was in order.

All set, the check-in agent assured Zwing and told her to prepare for longer wait times at security on Monday.

“My boyfriend and I are super excited and are counting down the hours til we’re finally reunited again! He keeps telling me that he can barely sleep out of excitement,” Zwing said Sunday via email. “For me it’s still so surreal that it’s actually about to happen! So I’ll believe it when I land in Chicago.”

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.https://b0bafcc8bdaa22c7d78b790f7036dda5.safeframe.googlesyndication.com/safeframe/1-0-38/html/container.html

A gradual recovery seems to be in the cards for international air travel. According to figures from aviation analytics company OAG, the number of filled seats arriving in the US from Europe this December is projected to be about 67% of the December 2019 level.

Airlines are still bringing back employees and aircraft sidelined because of the pandemic. Staffing issues, which could be exacerbated by employee vaccine mandates, have complicated operations for some carriers.

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 in the coming weeks.

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. Los Angeles’ indoor vaccination requirement goes into effect on November 8.

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.

New York, Miami and Los Angeles may see some of the most significant influxes of international visitors. They are travel app Hopper’s top destinations for foreign travelers this holiday season.

Those spots line up with client demand at Trailfinders in the United Kingdom, where the US is back to being the travel company’s top destination. New York, California and Florida lead the way in bookings, according to Nikki Davies, public relations director at Trailfinders.

Before you go

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

Children under 18 are exempt from the vaccination requirement. The CDC has the full slate of air travel requirements on its website.

Travelers must meet CDC criteria for being “fully vaccinated.” Paper and digital documentation are acceptable. Airlines are responsible for gathering and verifying this information from air travelers.

More information on documentation is on the CDC’s site.

For air travel

Air travelers also need a negative Covid-19 test. Testing is required of all fully vaccinated air travelers ages 2 and up, regardless of nationality. Passengers are required to test negative for Covid-19 within three days of their flight’s departure for the United States.

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.

Most Delta customers on international routes bound for the US can directly upload and verify their proof of vaccination status using the Delta FlyReady tool.

British Airways’ VeriFly mobile digital health pass works on all BA’s flights to the US, the airline confirmed.

Hard copies are a good idea, too.

“I would definitely make sure I have paper copies … of my passport and vaccination card and major credit card, kept separate from my wallet/handbag. And have digital copies of the same on my phone and emailed to my email account in case my phone/handbag/backpack/etc. gets lost or stolen,” said Kathleen Bangs, a former airline pilot who is a spokeswoman for flight tracking site FlightAware.

“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.

“Trade and travel facilitation remain a priority,” a Department of Homeland Security Q&A about the new policy says. “However, we cannot compromise national security which is our primary mission.”

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.”