Under a CDC regulation, airlines must comply with these rules in order to be allowed to disembark passengers in the United States.

The CDC dictates that negative results must come from a test that can detect persistent infection by ingesting parts of the pathogen itself. Two types of tests fall into this category: molecular tests (including PCR tests) and antigen tests. (Antibody tests that can only tell if someone has been infected in the past don’t count.)

Molecular tests look for segments of the genetic material or RNA of the virus. The most common molecular tests are based on a proven technique called the polymerase chain reaction or PCR – a gold standard in diagnosing infectious diseases. PCR testing can be expensive, and because samples must be passed through laboratories, it can take a few days for results to be returned. Experts say it is a good idea to plan ahead if you decide to take this type of test.

There are some rapid molecular tests that can be done in a doctor’s office from start to finish in a matter of minutes. That includes Abbott’s ID Now test. They’re considered less accurate than PCR-based tests, but get faster responses.

Antigen tests look for pieces of coronavirus proteins or antigens. They’re usually less accurate than molecular tests and have a harder time finding the virus when it’s scarce. However, most antigen tests can be done very quickly and cheaply, with results taking only a few minutes.

Certain antigen tests are only released for people with symptoms and may be more likely to provide inaccurate results when used to screen people who are feeling healthy.

Depending on the country travelers are leaving from, certain tests may not be available – and as a result, these new rules are likely to make it significantly more difficult to travel to the United States. Tests are usually offered through health care providers or community test centers, which can be set up through tourism offices and local health care providers. Some airports, such as Heathrow in London, offer on-site coronavirus testing. Some airlines like American, Jet Blue and United offer their customers in certain countries to arrange tests. For example, Delta has worked with the Mayo Clinic and national health authorities in several countries to simplify the testing and travel process.