People traveling immediately after the holiday may face uncertainty: Many private testing clinics and labs are closed on Christmas Day, so testing within the 72-hour window may prove difficult, especially for the P.C.R. screening, which must be sent to a lab and can take several days to process.
The rapid antigen test, a relatively new tool to detect the virus, gives a result in around 30 minutes, but it is not as widely available, although it is cheaper. Heathrow Airport, for example, charges passengers about $130 for P.C.R. results with 48 hours and about $60 for antigen tests with results within 45 minutes.
Both tests are offered at major British airports — including Heathrow and Gatwick, London’s two major hubs, and Manchester Airport — but passengers must register in advance. It was unclear how many would be able to procure a test and get a result in time for travel.
The introduction of new travel restrictions led to concerns that travelers to the United States would flock to the airport, as Londoners did at train stations last Saturday when tighter domestic rules were announced. But employees at Heathrow on Friday described a normal, if quieter, stream of passengers typical of Christmas Day, with most appearing to travel on long-haul flights.
Confused by the terms about coronavirus testing? Let us help:
- Antibody: A protein produced by the immune system that can recognize and attach precisely to specific kinds of viruses, bacteria, or other invaders.
- Antibody test/serology test: A test that detects antibodies specific to the coronavirus. Antibodies begin to appear in the blood about a week after the coronavirus has infected the body. Because antibodies take so long to develop, an antibody test can’t reliably diagnose an ongoing infection. But it can identify people who have been exposed to the coronavirus in the past.
- Antigen test: This test detects bits of coronavirus proteins called antigens. Antigen tests are fast, taking as little as five minutes, but are less accurate than tests that detect genetic material from the virus.
- Coronavirus: Any virus that belongs to the Orthocoronavirinae family of viruses. The coronavirus that causes Covid-19 is known as SARS-CoV-2.
- Covid-19: The disease caused by the new coronavirus. The name is short for coronavirus disease 2019.
- Isolation and quarantine: Isolation is the separation of people who know they are sick with a contagious disease from those who are not sick. Quarantine refers to restricting the movement of people who have been exposed to a virus.
- Nasopharyngeal swab: A long, flexible stick, tipped with a soft swab, that is inserted deep into the nose to get samples from the space where the nasal cavity meets the throat. Samples for coronavirus tests can also be collected with swabs that do not go as deep into the nose — sometimes called nasal swabs — or oral or throat swabs.
- Polymerase Chain Reaction (PCR): Scientists use PCR to make millions of copies of genetic material in a sample. Tests that use PCR enable researchers to detect the coronavirus even when it is scarce.
- Viral load: The amount of virus in a person’s body. In people infected by the coronavirus, the viral load may peak before they start to show symptoms, if symptoms appear at all.
Several airlines had already announced policies requiring proof of a negative test after a demand from Gov. Andrew M. Cuomo of New York that passengers arriving from London to John F. Kennedy International Airport would need to provide documentation of a negative test result.
“We can’t let history repeat itself with this new variant,” Mr. Cuomo had written on Twitter.
Also on Thursday, Gov. Phil Murphy of New Jersey said that passengers arriving at Newark Airport would need negative tests within 72 hours of departure to enter.
The American travel requirements are less draconian than those of other countries in Europe and Asia, which barred all travelers from Britain after the new coronavirus variant emerged. Experts are skeptical that travel bans can stop the spread of the variant. In fact, Dr. Anthony S. Fauci, the top U.S. infectious disease expert, said there was a good chance that the variant was already in the country.