For the near future, a passport or I.D. won't be enough to get you across many world borders. You also need to present a Covid-19 test with a negative result—and usually, it must be obtained 48–72 hours before arrival.
But which kind of coronavirus test is the best for meeting travel requirements?
First, it's helpful to understand what each test does.
The most accurate is the molecular test. This diagnostic exam, which flags active infection, includes the RT-PCR, LAMP, and nucleic acid amplification tests. Sometimes a sample is taken from the throat behind the nose, and sometimes saliva will do. Results can come back the same day, but usually patients have to wait a day or two.
The antigen test, on the other hand, can deliver results very quickly—sometimes within the hour. However, this test is less accurate than the molecular/PCR alternative. False positives are not uncommon with antigen tests, and they've shown a higher chance of missing an active infection. Doctors will often follow up a result with a PCR test just to make sure. Still, people tend to use antigen tests because they're fast.
Finally, there's the blood antibody test. This test looks for antibodies that the immune system produces to fight Covid-19. Those antibodies can take weeks after infection to show up in detectable numbers. They also remain in the blood for a while after infection is over, so an antibody test is not effective for detecting an active infection—only for indicating there was one in the recent past.
For more detailed explanations of each test type, their uses, and their limitations, the Food and Drug Administration has posted an information page.
So which one should you take if you want the best chance of crossing a border successfully?
The answer is the molecular test.
If you peruse our rundown of what various Caribbean countries require from would-be tourists, over and over you'll see the PCR test is the one that's demanded. That's a molecular virus test, and it can be conducted by nasal swab or by saliva.
Not all places want a PCR test. Hawaii, for example, insists upon a Nucleic Acid Amplification Test (NAAT). That's still a molecular test—but it's different than PCR.
Because different places demand different types of tests, it's imperative that you check ahead with authorities to find out which specific one is required.
The second wrinkle: Countries want to know visitors carry no active infection, so usually the molecular test must be no older than a few days, but each destination has its own preferred time limit.
For the most secure result, do not rely on an antigen test for entry to any country. Rely on a molecular test, and make sure you obtain results within the time window demanded by your destination.