(CNN) – The United States opened its doors to vaccinated international travelers on Monday, welcoming many visitors who have been banned from the country for nearly 20 months.

It was a precious opportunity for Maria Giribet Caldentey to see her twin grandchildren in California. She had to postpone this trip because the border was closed.

“I go on the first day they allow me to go … I have butterflies. I had some anxiety, but I’m happy to see them,” she told CNN at Paris’ Charles de Gaulle Airport .

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

However, with new requirements for U.S. air, land and ferry arrivals going into effect, the introduction of the rules is sure to create congestion.

“It gets a bit sloppy at first, I can assure you,” said Ed Bastian, CEO of Delta Air Lines, at a travel industry conference in late October. “Unfortunately there will be queues,” he said, citing “a rush of trips at a time”.

According to Delta spokesman Morgan Durrant, many Delta flights that were supposed to arrive on Monday were 100% booked out in the following weeks with high occupancy factors. Delta has seen international bookings grow 450% in the six weeks since the US reopening announcement, he said.

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

Spencer Platt / Getty Images

The Lufthansa Group told CNN that its airlines operated 31 flights to the US on Monday, all of which were fully booked. Virgin Atlantic reported that its flights were 98% busy, while both Swiss Air and British Airways reported high passenger traffic on flights departing to the United States.

A gradual recovery seems to be in sight for international air traffic. According to figures from the aerospace analysis company OAG, the number of occupied seats entering the US from Europe in December is projected to be around 67% of the December 2019 level.Airlines are still bringing back workers and planes that have been marginalized because of the pandemic. Have personnel problems that could be exacerbated by vaccination mandates for employees complicated operations for some airlines.

Customs and Border Protection (CBP) and the Transportation Security Administration are also short on staff and, shortly before Thanksgiving, face an impending deadline for federal employee vaccinations.

At busy land border crossings from Canada and Mexico, CBP expects longer waiting times.

Most of all, preparation and patience will be critical for international travelers heading to the United States in the coming weeks.

Here’s what travelers can expect and prepare for:

When you get there

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

Mask requirements exist in Hawaii, Illinois, New Mexico, Nevada, Oregon and Washington, for example. Washington, DC, and Puerto Rico also require masks in indoor public spaces.

Some cities, including New York and San Francisco, have vaccination requirements for indoor public spaces, including restaurants. The indoor vaccination in Los Angeles went into effect on November 8th.

Hawaii, which had some of the strictest entry requirements in the US, will now adapt to the new federal rules for international air travel. Although capacity restrictions in the country are being relaxed, there are still some restrictions.

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

Daniel Slim / AFP via Getty Images

New York, Miami, and Los Angeles have some of the largest influxes of international visitors. They are travel app Hoppers Top destinations for overseas travelers this holiday season.These spots meet customer demand at Trail finder in the UK, where the US is once again the travel company’s top destination. New York, California and Florida lead the bookings, according to Nikki Davies, PR director at Trailfinders.

Before you go

Vaccination is an important requirement for the vast majority of international travelers planning to travel to the United States.

Travelers must meet CDC criteria to be “fully vaccinated.” Papers and digital documents are permitted. More information about documentation is on the CDC website.

Airlines are responsible for collecting and verifying this information from air travelers.

Visitors to the new Summit One Vanderbilt Observatory in New York will take a glass elevator to the top on October 21.

Visitors to the new Summit One Vanderbilt Observatory in New York will take a glass elevator to the top on October 21.

Timothy A. Clary / AFP via Getty Images

Anna Zwing, 28, flew from Germany to Chicago on Monday to see her boyfriend for the first time in more than two years. She checked in personally in Frankfurt for her Lufthansa flight on Sunday to make sure that she ESTA application and all of the new Covid documentation that she had vigilantly collected and double and triple checked was fine.

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

“My boyfriend and I are super excited and are counting the hours until we are finally reunited! He keeps telling me that he can hardly sleep because of the excitement,” said Zwing by email on Sunday. “It’s still so surreal to me that it’s actually going to happen! So I’ll believe it when I land in Chicago.”

What vaccines are accepted for entry into the US?

With different vaccines used around the world, many travelers wonder if their vaccination is valid for entry.

All FDA cleared and approved vaccines, as well as vaccines with a WHO emergency list, are accepted for entry into the United States.

As of November 8th, the vaccines approved for travelers are:

• Janssen / Johnson & Johnson
• Pfizer-BioNTech
• Modern
• AstraZeneca
• Covaxin
• Covischild
• BIBP / Sinopharm®
• Sinovac

People are taken into account “fully vaccinated” by the CDC two weeks after their second dose in a two-dose series or two weeks after a single-dose vaccination.

Mixed vaccinations are also accepted. Some participants in vaccine studies are also considered fully vaccinated.

For air travel

Air travelers also need a negative Covid-19 test. Test is required all fully vaccinated air travelers aged 2 and over, regardless of nationality. Passengers must take a negative Covid-19 test within three days of their flight departing for the United States.

Unvaccinated Americans and a very limited number of unvaccinated international travelers who are vaccinated-free are required to have a test within one day of leaving the United States.

Many airlines have mobile apps and portals on their websites in which vaccination and test information can be processed digitally.

Most Delta customers on international routes to the US can upload and check their vaccination status directly by using the Delta FlyReady Tool.A Delta Air Lines employee works on the departures level of Los Angeles International Airport in August.  Delta will operate 139 flights from international destinations to the United States on November 8th.

A Delta Air Lines employee works on the departures level of Los Angeles International Airport in August. Delta will operate 139 flights from international destinations to the United States on November 8th.

Mario Tama / Getty Images

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

Hard copies are also a good idea.

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

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

Miami (pictured here with Ocean Drive in Miami Beach) is one of the top three travel destinations for international travelers on Hopper.

Miami (pictured here with Ocean Drive in Miami Beach) is one of the top three travel destinations for international travelers on Hopper.

Joe Raedle / Getty Images

At land borders

Customs and Border Protection anticipates an increase in the volume of travelers and waiting times for land and ferry crossings and encourages travelers to have their identification and vaccination documents ready. The agency also encourages travelers to do their CBP One App and Facial biometrics.

The Department of Homeland Security said in a press release on Monday that it would work to avoid long waiting times.

“However, long lines are expected in the first few days of catching up and CBP will adjust resources as needed while continuing to facilitate legitimate trade and travel and protect our national security,” the agency said.

Digital and paper documents are acceptable for proof of vaccination and vaccination cards do not need to be in English.

Travelers should be ready to certify their vaccination status and reason for travel. You should also be ready to provide a full vaccination certificate if requested by a CBP officer.

Children under the age of 18 traveling with vaccinated adults are exempt from vaccination requirements.

Covid tests are not required for land and ferry crossings.

The web of rules and requirements for traveling internationally is undeniably confused.

“Just getting on a plane and going somewhere doesn’t work anymore,” said travel advisor Dave Hershberger of Prestige Travel in Cincinnati, Ohio. “You must do your homework.”

CNN’s Joseph Ataman, Chris Liakos, Anna Cooban, Chris Isidore and Geneva Sands contributed to this report.