As countries around the world react to the new and little-understood omicron variant of the coronavirus, travelers are once again facing unexpected border closures, evolving entry rules and a dark cloud of potential cancellations.

The United States on Wednesday joined the list of more than 20 countries that detected cases of the new variant, health authorities said. It wasn’t immediately clear if that would trigger new restrictions for Americans traveling to other countries.

According to reporting from The Washington Post, U.S. officials were planning to lay out new requirements later this week that will ratchet up testing — and potentially require a controversial quarantine period — for anyone coming back into the country. That would add wrinkles to already complicated foreign travel plans.

Here are nine key questions and answers for travelers in light of the latest developments.

Should I cancel my trip abroad?

The World Health Organization warned this week that some groups of people should postpone travel to areas with community spread of the virus. That includes people who are unvaccinated or who have not previously been infected, those 60 or older, or anyone with preexisting medical conditions that put them at greater risk of severe illness. Epidemiologist Chris Beyrer told The Post on Friday that he wanted to see how effective vaccines are against the new variant before deciding whether to nix a work trip to South Africa.

Are there any new restrictions on domestic travel?

As of Wednesday afternoon, domestic travel was carrying on as normal — or at least covid-era normal. That means masks are required in airports and on planes, trains, and other forms of public transportation. Some locations may emphasize mask-wearing; New York City, for example, is strongly recommending that people wear masks indoors in public places.

What do I have to do if I’m returning to the U.S. from a foreign country?

Effective Monday, Dec. 6, all travelers flying into the United States have to take a test one day before boarding, regardless of nationality or vaccination status.

Before announcing that change — a step up from a requirement for testing within three days of a flight — U.S. officials told the The Washington Post they were considering whether they would require passengers to get tested again within three to five days of arriving. They were also discussing the possibility of a seven-day self-quarantine measure. When asked what other measures the country might take, White House press secretary Jen Psaki told reporters Thursday that “nothing is off the table.”

What do I have to do if I’m leaving the country?

Border rules vary according to destination — and travelers should be aware those rules could change fast. The United Kingdom, for example, recently announced that anyone coming to the country will need to self-isolate until they get a negative PCR result. They will have to take the test by the end of their second day in the country. Canada is also requiring air travelers to test upon arrival and quarantine until they have results, though anyone coming from the United States is exempt.

Is anywhere completely off limits?

A small number of countries have put broad travel bans in place. As of Sunday, noncitizens are not allowed to visit Israel for two weeks. Morocco is suspending incoming air travel for two weeks as of Monday, the Associated Press reported. And Japan is keeping all nonresident foreigners out of the country for at least a month.

Many other countries have banned noncitizens coming from certain countries in southern Africa. According to the World Health Organization, 56 countries had put some travel restrictions in place as of Sunday.

Where has the omicron variant been detected?

As of Wednesday, the variant had been found in more than 20 countries in North America, South America, Europe, Africa, the Middle East, Asia and Australia. The first U.S. case was identified Wednesday in someone in California who had returned from South Africa on Nov. 22.

How can I keep track of rule changes?

Airlines are supposed to keep travelers updated on the rules they need to follow in their destinations. And the U.S. State Department urges travelers to check embassy websites and the department’s website (travel.state.gov) for information such as entry and exit rules, testing availability and quarantine information.

The department said U.S. citizens going overseas should also register their plans in the Smart Traveler Enrollment Program.

Do travel restrictions work?

The World Health Organization has been critical of measures to ban travel from southern Africa, where the variant was first detected. And public health experts have pointed out that those border closures come too late to contain a new variant. But officials have said they hope restricting travel will help slow the spread while scientists determine how transmissible omicron is and how effective vaccines are against it.

How will the tourism industry be affected?

Even before the emergence of the omicron variant, the estimated blow to world tourism was $1.6 trillion this year, according to the U.N. World Tourism Organization. Last year’s direct economic hit to tourism was about $2 trillion. The latest variant comes as travel was starting to rebound in many parts of the world, signaling a new round of financial woes.

Ellen Francis, Lena H. Sun and Tyler Pager contributed to this report.