With hundreds of millions of vaccine doses distributed, the need for coronavirus testing is changing in America.

The Centers for Disease Control and Prevention has said fully vaccinated people can travel with less risk to their health and don’t need to get tested before or after travel, unless their destination requires it or covid-19 symptoms emerge after the trip. However Americans flying abroad must provide a negative coronavirus test to the airline before flying home, even if they are fully vaccinated. Some just want a test for peace of mind.

Here’s testing advice from health experts for would-be travelers.

Who needs to get a test to travel?

Vaccine passports could largely replace the need for showing negative coronavirus results to cross certain borders, but until that’s official, you may still need to get tested to travel.

For those who are not vaccinated, the CDC guidance on coronavirus testing for travel remains the same: Get tested with a viral test one to three days before your trip, and get tested with a viral test three to five days after travel (plus stay home and self-quarantine for a week after travel).

Where to find a coronavirus test before (or after) you travel

There’s a variety of places offering coronavirus testing, including urgent care centers, travel clinics, fire stations, pop-up sites, most hospitals, pharmacies and some airports.

Lin Chen, a doctor and director of the Travel Medicine Center at Mount Auburn Hospital in Cambridge, Mass., says potential travelers should check in with their primary care provider, who may know the best options for testing in their area. Other options include looking at city and state health department websites for testing resources. If you’re going abroad, you can check the U.S. Embassy website of the country they’re visiting for applicable testing information.

While some people may have success walking into testing sites without an appointment, Chen advises against waiting until the last minute to arrange your test.

If you can’t find an in-person testing site, you can order one to be delivered to your house. One such option is Pixel, an at-home nasal swab collection kit from LabCorp for individuals 18 years or older who meet criteria to warrant a test. The FDA-authorized RT-PCR kit arrives in about two days, and results are returned within a few days.

Another option is Abbott’s BinaxNOW kit, which promises results in 15 minutes. You can find it at pharmacies, or order one online that comes with real-time supervision from a telehealth service to meet requirements for international travel.

Depending on the site, and whether a traveler has insurance, the price of a coronavirus test will vary.

What kind of coronavirus test to expect

If you’re getting a coronavirus test to meet a travel restriction requirement, make sure you know which type you need. Viral tests are designed to detect a current infection, while an antibody test will tell you if you’ve had the virus in the past.

“There are so many different kinds of tests … and there’s also so many variables that would make somebody test positive or negative that it’s just ripe for confusion,” says Gigi Gronvall, a senior scholar at the Johns Hopkins Center for Health Security.

Opt for a viral, or diagnostic, option like the RT-PCR test to find out whether you actively have the coronavirus before traveling. This can be administered by nasal or throat swabs, as well as by taking saliva samples.

“The PCR test tends to be more sensitive,” Gronvall says. “That means they pick up lower amounts of virus. … But the downside, of course, is that it usually takes longer to get results.”

Another option is getting a rapid diagnostic test, with results ready in as little as 15 minutes. There are two types of rapid tests, molecular and antigen, and the latter has come under fire for its questionable accuracy.

“Rapid antigen tests tend to be a little less sensitive,” Gronvall says. “They tend to miss some people who are actually infectious.”

A test’s efficacy can also depend on the level of infectiousness of the traveler. “If you are very infectious, you tend to have a lot of virus in your nose, and so it’s easier for any of the tests to pick it up,” Gronvall explains.

How to time your coronavirus test for a trip

Travelers need to find a sweet spot for their coronavirus testing before a trip. Plan it too early, and you have more time to get the virus before your trip and nullify your test result. Plan your test too close to your departure date, and if there’s a delay getting your results back, you’ll risk spreading the virus or being denied entry to a destination.

As you plan your test strategy, note that different tests will come with different wait times for results. Some RT-PCR test results can come back in two days. Rapid tests can return results in minutes. On the other end of the spectrum, some people have waited weeks.

Even if you’re promised a timeline for your results, there may be a delay, which may be a bigger issue for those attempting to travel to a destination that requires proof of a negative test.

“I’ve certainly seen that in some of my travelers: a lot of anxiety about the right timing for the test and getting the test scheduled,” says Chen, who is also president of the International Society of Travel Medicine.

Having a flexible travel schedule, or planning some buffer time between when your test results return and the start of your trip, will help alleviate that timing anxiety.

Should you get tested before, during or after a trip?

If you’re not vaccinated, getting tested before a trip could help you avoid spreading the virus. According to a CDC spokesperson, “The pre-travel testing would reduce the risk of allowing COVID-19 infected people on airplanes and other forms of public transportation, provided that the results of the testing are known and acted upon before travel begins.”

Getting a test during, at the end or after a trip can also help you avoid spreading the virus while you’re traveling or at home. A post-arrival test may also be recommended by your state’s coronavirus task force.

“Post-arrival testing can help reduce the risk of infected persons spreading the virus at their destination (whether at home or at their travel location),” the CDC spokesperson said.

In November, the CDC released guidelines recommending non-vaccinated domestic and international travelers get tested one to three days before departure and three to five days after travel. They also recommend quarantining for seven days upon return, even if your test is negative.

A negative test doesn’t guarantee your safety while you travel

As the CDC says, a negative coronavirus test result means you didn’t have the virus when you took the test (or you were tested too early in the infection for the virus to be detected). It doesn’t protect you from future infection after you leave the testing site and take a plane, train, bus or car on a trip.

And Chen warns would-be travelers that coronavirus tests aren’t always accurate.

“Some people will turn out to be falsely negative even when they do carry the virus,” she says. “I think it’s a little bit reassuring if somebody tests negative, but it’s not an absolute guarantee.”