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It's the worst-case scenario that you don't really think about.

After a three-week vacation in Greece, a hotel art consultant from Miami and her husband took pre-departure coronaviruses tests required to fly home from overseas. He did not test positive.

The United States still requires all international air passengers to present a negative test within one day of departure, despite the fact that coronaviruses travel restrictions have been lifted in many parts of the world. Ms. Fishman was supposed to be isolated and delayed travel for 10 days, but she had to go to a wedding earlier.

She took advantage of a quirk in the rules that allowed her to go home after five days. After spending a night in Toronto, Ms. Fishman and her husband drove across the border into Buffalo to catch a flight home. There is no testing for flying domestically.

I didn't have any symptoms, no fever, nothing. When I am traveling, the C.D.C. says you can end isolation after five days if you test positive. That should be illegal.

Airlines don't require passengers to provide reasons for canceling or changing flights, so it's not clear how many people are using back doors to get home. Some travelers and travel advisers are sharing ideas on how to avoid getting stuck.

Ms. Fishman said she was following the instructions of a family friend who used a different route to get home after he tested positive in France.

Ms. Fishman pointed to the C.D.C.'s guidance for people who catch the virus in the United States, which she said she was worried about. The C.D.C. said that the majority of coronavirus transmission occurs early in the illness.

I slept in the same bed as my husband for five nights and he didn't catch it, so I don't think I was contagious when I got home.

The C.D.C. did not say why it has different policies for Americans who test positive at home and abroad, but a spokeswoman for the agency reiterated that travelers should follow the 10-day guidance to not travel before boarding a flight to the United States, even if they test negative. The Department of Homeland Security wouldn't comment on the possibility of travelers using land borders to circumvent the testing requirement for air travel.

When fewer than 10 percent of Americans werevaccinated, the United States introduced the testing requirement in January of 2021. Many American travelers, as well as industry representatives, are calling for the requirement to be lifted, arguing that it does little to prevent new variant of the virus from spreading in the United States.

The American Society of Travel Advisors said that the current inbound testing policy is nothing short of useless.

David Freedman, president-elect of the American Society of Tropical Medicine and Hygiene, is concerned by travelers using this method, warning that infectious people taking a flight or public transportation to a border crossing will put a lot of people who are not up-to-date on their vaccinations at risk.

The infectious person may be carrying one of the new variant BA. 4 or a degree. He said that there may be new enhanced vaccines by the fall that may be bad.

One American traveler and her wife decided to return to Seattle via Canada because a U.S.- Canada border crossing was close to their home. She said she would drive home and isolated if she had to take a test at the airport. The woman wanted to speak out because she was afraid of repercussions.

We had symptoms for about two weeks before testing positive. She said that they felt confident to travel after they tested positive. No one will pull you out of a crowd if you look healthy, determined and at ease.

Canadian officials often ask passengers health questions, and some airports, like those in Toronto and Vancouver, may randomly select some passengers to take coronaviruses tests before being cleared to leave the airport.

If you have Covid-19 symptoms within 10 days of your departure, you won't be allowed to board a flight into Canada. Travelers have to fill out a health and travel form before they can travel to Canada. The form has questions about vaccination status and whether a passenger has symptoms of coronaviruses.

All travelers arriving in Canada are required to answer all questions honestly, according to Rebecca Purdy, a senior spokeswoman for the Canada Border Services Agency.

Travelers who knowingly travel to Canada with an active Covid-19 infection and who do not disclose this information may face penalties and/or criminal charges and foreign nationals may also be denied entry and/or banned from returning to Canada.

Requests for comment from Mexican authorities were not returned. Travelers from the U.S. can enter Mexico without being tested or subject to health screenings.

People are using the back doors to ensure they can travel home on time despite the expense.

Hilary Aranda had just finished a two-week dance tour in Italy when her group of 12 Americans tested positive for a drug. She canceled her flight home to Los Angeles for a flight to Mexico with stops in London and Mexico City to avoid a positive result and headaches. She drove home after crossing the land border. The changes to her itinerary cost her more than $2,000.

Ms. Aranda knew that if she took the test, it would come out positive and she didn't want to risk it.

Travelers who are more risk-averse, but still determined to avoid isolation in another country, have been using Quick MD to obtain documentation of recovery, which allows them to travel to the United States without having to show a negative test. It can take weeks or even months before some people test negative for a positive test, so the option is available to travelers who test positive 10 days after their initial positive test.

Some travelers lie about when their symptoms started so that they can return home without having to go through the 10-day isolation period.

One traveler who asked to speak out of fear of getting into trouble with authorities said that he told the doctor that his symptoms started earlier than they did. He tested positive in London a day before his flight to New York, but returned home three days later.

He said it was so easy to get his clearance document.

Quick MD didn't respond to many requests for comment.

Ms. Fishman made it back home in time for the wedding and never developed Covid-19 symptoms, although she blamed the stress caused by trying to get back in time.

She said that the testing requirement is useless because her chances of catching Covid in Miami are just as high.

Travelers are warned that they can be caught out. If you violate instructions upon entering Canada, you could be sentenced to up to six months in prison or a fine of up to 600,000 Canadian dollars.