So you’ve been vaccinated. Congratulations! As per the Centers for Disease Control and Prevention (CDC) guidelines, you can gather with other immunized people without masks or social distancing, and even travel internationally. But what happens if the friend you met up with for lunch last weekend tests positive for COVID-19? Do you need to get swabbed, too?
The basic recommendation from the CDC is that fully vaccinated individuals—those who received their last dose two or more weeks ago—shouldn’t get tested for the coronavirus unless they have symptoms of COVID-19. But the country is requiring everyone who travels abroad to have a negative test before boarding a return flight to the US. The CDC also advises to get tested again three to five days after arrival.
But there are many scenarios that the guidelines don’t address.
Monica Gandhi, an infectious disease specialist at the University of California, San Francisco, mostly agrees with the CDC guidelines. But she says it’s pointless for asymptomatic vaccinated people to get tested, given that they’re unlikely to spread or get sick from the virus.
The foundation of Gandhi’s stance is two published studies and one preprint showing that vaccinated people who’ve been exposed to COVID-19 have fewer viral particles in their nose compared to unvaccinated people. The more virus in a person’s nose, the more likely they are to be infectious, according to a study of 282 COVID-19 patients and their contacts in Catalonia, Spain.
But if a vaccinated person does have symptoms, they should get tested. “Confirm for their own sake that they don’t have COVID. Because we have treatments that should be offered if they do,” Gandhi says.
There are also some rare cases where vaccinated people who don’t have symptoms may want to get tested. One is if they’re visiting an unvaccinated individual at high risk of severe COVID-19. “I would personally be comfortable visiting an unvaccinated older person based on the real-world effectiveness of the vaccine,” Gandhi says. “But if you want to be absolutely the safest that you can ever be, I think you can test before you go.”
[Related: How long will we keep wearing masks?]
In all other scenarios, testing asymptomatic people who’ve been vaccinated isn’t necessary, even if they work somewhere that has an outbreak or are attending a big event like a concert. (But if you’re required to get tested, you should comply). “I’m concerned about the impact for a healthy person who’s asymptomatic to be told they have COVID, when they may not,” Gandhi says. “I’m concerned about implications for quarantine, for anxiety, for stress.”
Families have an extra set of questions to consider because of their unvaccinated kids. If you’re going to travel with kids either inside or outside the country, they should get tested one to three days before the trip, according to the CDC. Children who are exposed to someone with COVID-19 should follow the guidelines and get tested and quarantine at home for 14 days. But even if there’s an outbreak at school, vaccinated guardians don’t need to get swabbed, says Gigi Gronvall, an immunologist at the Johns Hopkins Center for Health Security.
These recommendations only apply to fully vaccinated people, however. If you aren’t two weeks out from your final dose, act as if you can still catch and spread the virus, Gandhi says. Although a single dose of the Moderna and Pfizer vaccines provokes a strong immune response, some people do get infected between the first and second shots, so err on the side of caution.
Trials for the Moderna and Pfizer vaccines suggest that they offer protection for at least six months, but it’s unclear just how long immunity will last. It can also vary from person to person. So stay vigilant about symptoms and get tested and quarantine immediately if you experience them.
In general, PCR tests are considered the gold standard for diagnosing COVID-19 because they’re slightly more sensitive than antigen tests. But they also run the risk of turning up positive when they detect even a small amount of the virus. Some experts think that people are infectious if they have viral loads of a cycle threshold (Ct) of 30 or less. But most PCR tests run for 40 Ct, so they could report COVID-19 in people who aren’t even infectious.
For this reason, Gandhi recommends that vaccinated people opt for rapid antigen tests, which are now widely available and return results in a matter of minutes. If they do get a PCR test, they should choose one that lists the Ct value. If that number is above 35, their viral load is low, and the person is probably safe around others. But if the number is below 35, treat it as a positive test result.
The vaccine and the antibodies shouldn’t affect the accuracy of test results. What determines false positive and negative rates is the test itself and how much community transmission there is in your area, Gronvall says. “It’s not going to be a function of whether or not you’re vaccinated.”
COVID testing has slowed in recent months, but results are still important both for individuals and public health. Identifying people with COVID-19 helps contact tracers determine who’s been exposed and needs to self-isolate. Pinpointing infected people also allows them to receive COVID-19 treatments that can aid in their recovery.
On the public health level, testing is crucial for naming and tracking new variants. It’s especially important to learn what variant a symptomatic vaccinated person is carrying so researchers can investigate whether the antibodies are still effective against it.
Once COVID is less prevalent, testing won’t be as big of a deal, Gandhi says. If you go to the doctor with a respiratory illness, the virus will just be one of the smattering of diseases they test you for. But that won’t happen until approximately 70 to 90 percent of the US population is vaccinated and the country reaches herd immunity. “We’re still not out of the woods, Gandhi says. “And that’s why we’re still testing.”