Tests that detect current infections with SARS-CoV-2, the virus that causes COVID-19, are known as viral tests. There are two types: a Nucleic Acid Amplification Test, or NAAT, and an antigen test.
Many of the NAATs use a molecular biology technique known as the polymerase chain reaction, or PCR, to detect even a very tiny amount of the virus in a specimen.
The PCR test takes advantage of some natural features of biology to essentially scan through all of the RNA present in a sample — such as a nasal swab — and search for the presence of coronavirus RNA.
The Centers for Disease Control and Prevention says NAATs “are unlikely to return a false-negative result of SARS-CoV-2,” and it recommends only laboratory-based NAATS, the most sensitive tests, to confirm infection. It also says saliva tests aren’t as optimal as those using swabs of the nose or nasopharynx (upper throat behind the nose).
An antigen test is designed to detect the coronavirus antigen, a structure on the surface of the virus that triggers an immune response. Also conducted via a nasal or throat swab, antigen tests are less sensitive than NAATs but inexpensive and can usually produce results in about 15 minutes. Getting a result from a NAAT test can take about an hour or even a few days.
Testing can help stop the spread of the coronavirus from those who test positive, because they can then isolate themselves from others. But, as we’ve explained, testing has its limitations. It can take a few days or even longer after exposure to the virus before it’s detectable by the diagnostic tests. So even some with negative results could be infectious.
Still, to help limit the spread, the U.S. has instituted travel requirements and recommendations. All international airline passengers, including U.S. citizens and residents, and those who are fully vaccinated, arriving in the U.S. must show “a negative COVID-19 test, taken within three (3) calendar days of departure, or proof of recovery from the virus within the last 90 days,” as the State Department explains.
The CDC also recommends that everyone, including the fully vaccinated, get a viral test three to five days after returning from international travel.
The CDC recommends that those who aren’t fully vaccinated avoid international travel, and if they must travel, to get a test one to three days beforehand. The unvaccinated also should self-quarantine for seven days after returning to the U.S., even if they tested negative before or upon returning.
International destinations may have their own testing requirements as well.
As for domestic travel, the fully vaccinated don’t need to get tested, but the CDC recommends the unvaccinated follow the same guidelines as those for international travel.