Beginning Jan. 26, international travelers bound for the United States must show negative coronavirus test results before boarding their flights. Here’s what you need to know.

Credit…Eduardo Munoz/Reuters

Starting Jan. 26, international travelers arriving in the United States, including Americans returning from abroad, will need to show negative coronavirus test results before boarding their flights — the latest in a wave of policies designed to curb the calamitous spread of Covid-19.

The Centers for Disease Control and Prevention announced the new orders on Tuesday. In a statement, the C.D.C. director Dr. Robert R. Redfield stressed that testing alone cannot “eliminate all risk,” urging travelers to continue wearing masks, maintain physical distance and follow quarantine guidelines.

Travel — whether it’s by plane, train or automobile — remains ill-advised, experts say, especially as researchers continue to identify new and potentially more contagious variants of the virus. The best way for an infectious microbe to move is to hitch a ride atop a wayfaring human. And train stations, airports and other travel hubs, where large groups inevitably gather, only create more conduits for the coronavirus to traverse.

“Unless you have a really good reason to travel, don’t,” said Dr. Krutika Kuppalli, an infectious disease physician at the Medical University of South Carolina.

If you do plan to fly internationally into the United States, here’s what you need to know.

The C.D.C.’s full policy on travel recommends that trips be book ended by tests on both sides. Passengers on inbound flights to the United States (and flights connecting through the United States) will need to take a test that looks for the virus within the three days before their plane departs.

Jasmine Reed, a C.D.C. spokeswoman, said the agency does not require that international travelers undergo a mandatory federal quarantine. But travelers are urged to seek out another test three to five days after arrival, and limit their movements and interactions for at least seven days post-travel, even if their initial test is negative.

Not everyone who is infected by the coronavirus develops symptoms. Those who do tend to start feeling sick between two and 14 days after being exposed to the virus, and might not test positive until around the time their symptoms appear.

But it’s specifically the preflight negative results that green-light people to board. Passengers ages 2 and up will need to show documentation of a laboratory test result, in paper or electronic form that can be viewed on a smartphone, to an airline employee. Alternatively, people who have previously had a known case of Covid-19 can show official proof that they’ve recovered from their illness. Anyone who fails to provide these records, or declines to take a test, cannot board.

A positive test result is also an automatic no-go for travel. So are Covid-like symptoms, even if your test result comes back negative.

According to a C.D.C. order, airlines must comply with these rules to receive permission to disembark passengers in the United States.

The C.D.C. stipulates that negative results need to come out of a test that can detect an ongoing infection, by picking up on pieces of the pathogen itself. Two types of tests fall into this category: molecular tests (which include P.C.R. tests) and antigen tests. (Antibody tests, which can only determine whether someone was infected in the past, don’t count.)

Molecular tests search for segments of the virus’s genetic material, or RNA. The most common molecular tests rely on a tried-and-true technique called polymerase chain reaction, or P.C.R. — a gold standard in infectious disease diagnostics. P.C.R. tests can carry a hefty price tag, and, because they require samples to be routed through laboratories, they might take a few days to return results. Experts say it’s a good idea to plan ahead if you opt for this type of test.

There are a couple of rapid molecular tests that can run from start to finish in a doctor’s office in a matter of minutes. These include Abbott’s ID Now test. They’re considered less accurate than P.C.R.-based tests, but will get you answers faster.

Antigen tests hunt for pieces of coronavirus proteins, or antigens. They tend to be less accurate than molecular tests, and are worse at finding the virus when it’s scarce. But most antigen tests can be done very quickly and cheaply, taking only minutes to deliver results.

Certain antigen tests are cleared only for use for people with symptoms, and might more frequently deliver inaccurate results when used to screen people who feel healthy.

Depending on the country that travelers are departing from, certain tests might not be available — and, as a result, these new rules will likely make it a lot harder for people to enter the United States. Tests are generally offered through health care providers or community testing sites, which can be located via tourism bureaus and local health care providers. Some airports, such as Heathrow in London, are offering on-site coronavirus testing. And a few airlines, like American, Jet Blue and United, are offering to help their customers in certain countries arrange testing. Delta, for instance, has partnered with Mayo Clinic and national health authorities in multiple countries to ease the test-and-travel process.

But by and large, “it’s going to be very difficult to fulfill some of these requirements,” especially in parts of the world where the infrastructure for testing is already strained by Covid caseloads, said Paige Larkin, a clinical microbiologist at NorthShore University HealthSystem.

For people who have options, experts recommend a P.C.R. test, to maximize accuracy.

Many countries, including Canada, Britain and China, already require preboarding negatives. Some will only accept results from molecular tests, like P.C.R.-based tests, because they worry that antigen tests will miss a large percentage of infections.

No. It also can’t guarantee you won’t become newly infected between the time you take the test and receive the result — or between the time you take the test and depart on your flight.

A recent report published by the C.D.C. concluded that pre-departure testing is unlikely to completely quash transmission of the coronavirus in-flight, where quarters are close and masking is tough to maintain for long periods of time.

Dr. Valerie Fitzhugh, a pathologist at Rutgers University, said the best way to think about a negative test result is to consider it a single point in time, at which the virus wasn’t detected by a test. Some tests are less sensitive than others, and might miss the virus in an infected person — especially if that individual was only recently exposed, and isn’t yet carrying the coronavirus in high amounts. Every test also makes mistakes to some degree, and might ping back false negatives at a low rate.

Being too cavalier about a negative test result could imperil the people around you, Dr. Larkin said, especially on a long international flight. “It does not absolve you of the responsibility of wearing a mask on the plane,” she said.

And no test can predict the future.

“A negative today does not ensure negative tomorrow,” Dr. Fitzhugh said.

If people have to wait for their test results, Dr. Fitzhugh recommends quarantining in the meantime. “That is their best chance for staying negative before they board the plane,” she said.

C.D.C. guidelines also recommend being extra vigilant about avoiding gatherings, wearing masks, maintaining physical distance and practicing good hygiene in the two weeks leading up to a trip.

“A test creates a safety net,” said Dr. Bill Morice, president of Mayo Clinic Laboratories in Minnesota. “But it’s a safety net with pretty big holes.”

The C.D.C. offers one alternative to a negative test result. People who can prove they have already recovered from a coronavirus infection and can present a past positive test result, received within the last three months, paired with a letter from a health care provider that states they are clear to travel.

Part of the reason for this might be practical. Even after a person has recovered from a coronavirus infection and are no longer contagious to others, harmless shreds of RNA can linger in the body, triggering false positives on P.C.R. tests — sometimes for weeks or months. The C.D.C. recommends against getting tested again in the three months after a positive virus test, as long as symptoms have abated.

Dr. Kuppalli said this policy made her uneasy because people might occasionally get reinfected. Some might be falsely emboldened with the idea that they can’t catch the virus again, or pass it on to other people — neither of which is a guarantee.

Proof of recent vaccination, on the other hand, can’t qualify a person to travel without a negative test result. The vaccines given emergency clearance in the United States so far, developed by Pfizer and Moderna, were shown in clinical trials to be about 95 percent effective at preventing symptomatic cases of Covid-19. But researchers still aren’t sure about the degree to which they stop people from catching the virus and spreading it to others.

In recent months, dozens of countries have set test requirements to limit the movement of infected people across borders. The United States’ new policy is, in some ways, a very late comer.

Ms. Reed, of the C.D.C., said the agency’s new policy was implemented partly in response to reports of more transmissible variants of the virus emerging in other countries — several of which have already been inadvertently imported and exported across international lines. “With the U.S. already in surge status, the testing requirement for air passengers will help slow the spread of the virus as we work to vaccinate the American public,” she said.

In late December, the C.D.C. required inbound travelers from Britain, where one of the virus variants was identified, to show proof of a negative result on a coronavirus test. Dozens of cases of infections by the U.K. variant, as it has come to be known, have already been pinpointed in the United States.

Originally posted on https://www.nytimes.com/2021/01/14/travel/international-travel-covid-test.html