5 things to know about COVID-19 tests in the age of Omicron

The new variant is highly transmissible—but much else remains unknown. Here’s how scientists suggest using tests to stay safe.

With the holidays here and the threat from the Omicron variant growing larger, Americans now face a familiar dilemma: Should you go forward with your family gatherings or skip them, possibly for the second year in a row?

Even fully vaccinated Americans who believed the shots were the ticket to getting back to normal have been watching anxiously as the more transmissible Omicron has jumped from causing 12.6 percent to 73.2 percent of COVID-19 cases in just a week. Public health officials caution that breakthrough infections will account for some of those new cases—even though the vaccines still largely protect against severe disease and death.

But there’s a useful tool to help decide whether to gather over the holidays: testing. Rachael Piltch-Loeb, a researcher at the Harvard T.H. Chan School of Public Health, says that tests offer sorely needed insight even when there’s still much unknown about Omicron—including how likely it is for fully vaccinated people to get infected.

Barun Mathema, assistant professor of epidemiology at Columbia University Mailman School of Public Health, agrees. “Testing is the only way to really know what risk you pose to yourself and to your community,” he says. “Without it, essentially, you’re blind.”

Tests are in short supply right now, but the Biden administration has announced a plan to deliver a half billion free rapid tests to Americans who want them. Unfortunately, those tests won’t be available until January. In the meantime, here’s what you need to know about testing in the age of the Omicron variant—and what experts say you should do if you test positive.

1) How long should you wait to test after an exposure?

Every viral disease has an incubation period—the time from when you were exposed to a pathogen to when it can be detected. Currently, the Centers for Disease Control and Prevention recommends waiting five to seven days after exposure to COVID-19 before taking a test. Although that’s still the rule of thumb, Piltch-Loeb says it may change with the Omicron variant.

Some early data suggests that it takes only around two to three days for people to start showing symptoms after exposure to the Omicron variant. Likewise, Piltch-Loeb says that tests seem to be picking up the virus within about two days of exposure as well. Although that’s a bad sign for the transmissibility of the virus, the silver lining is that it may not take five days to get a positive result—allowing you to cancel future plans even sooner if necessary.

2) Which test should you use? Why? 

Polymerase chain reaction, or PCR, tests are considered the gold standard for detecting COVID-19. These tests—which look for the presence of genetic material from the virus—are highly sensitive and are unlikely to lead to false negatives.

Rapid antigen tests, meanwhile, are less reliable. They detect pieces of proteins from the virus and work best when you have a higher viral load, which is why they’re often used when people are symptomatic. Some brands of tests are more reliable than others, too: Piltch-Loeb suggests checking the CDC website, which lists the SARS-CoV-2 rapid antigen tests that have been authorized for emergency use in the U.S.

But given how quickly Omicron can spread, Piltch-Loeb says the more reliable PCR tests don’t always make sense. While it typically takes two to three days to get the lab results back, the recent surge in testing has overwhelmed laboratories, meaning that it is taking even longer for results to be returned. So if you don’t get your result back in time for a planned gathering, Piltch-Loeb says, the PCR may not help make a time-sensitive decision.

It’s also unclear how people who are fully vaccinated and boosted should interpret a PCR test. The CDC acknowledges that fully vaccinated people are infectious for less time than unvaccinated people.

That’s where the rapid antigen tests come in. Within 15 to 30 minutes, rapid tests provide a snapshot of whether you have a high load of virus at that time. So if you’re fully vaccinated, a positive rapid test may ultimately be a better indicator than a positive PCR test for whether you are shedding virus and pose a threat to grandma tonight.

Meanwhile, a negative rapid test likely means you are not infectious at that point in time—although the virus may still be incubating inside you. But experts caution that this isn’t a certainty given the potential for false negatives. Studies have shown that rapid tests are able to catch up to 85 percent of COVID-19 cases—meaning there’s plenty of room for error.

Ultimately, however, choosing a test comes down to your risk tolerance, Mathema says. He agrees that positive rapid tests are reliable, particularly when you are symptomatic, since false positives are far less likely than false negatives. However, he adds, “The part that I worry about is when you get a negative. What does the negative mean? What level of comfort does that give you?” If you want to be conscientious, he recommends getting the PCR test instead of, or in addition to, a rapid test.

3) If you test negative on a rapid test, what should you do?

If you have symptoms but test negative for COVID-19, you should keep testing. The CDC recommends taking two or more follow-up rapid tests, waiting at least 24 hours between each one. If you continue to test negative, Piltch-Loeb says, it’s likely you have a different kind of respiratory infection. But that’s not a guarantee. And since spreading the cold or the flu is generally frowned upon, she suggests isolating and masking until you’re symptom-free.

If you don’t have symptoms and test negative, however, Piltch-Loeb says to keep in mind that timing is critical for rapid tests. Since Omicron is so transmissible—and these tests provide only a snapshot in time—you could test negative in the morning and positive by the afternoon. To lower your chances of spreading the virus at Christmas dinner, you should take a rapid test an hour beforehand.

For true peace of mind, however, Mathema suggests following up a negative rapid test with a PCR test before gathering with friends and family—no matter your vaccination status.

4) If you test positive with the rapid test, what should you do? 

The CDC advises anyone with a positive test to stay home or isolate for 10 days and wear a mask if you’re around others. You should also notify anyone that you’ve had close contact with in the last two days and alert your doctor. People who are unvaccinated should adhere to this rule since they are more likely to become infected and spread the virus.

If you are vaccinated and don’t have any symptoms, however, Piltch-Loeb suggests following up with a second rapid test later that day or the next. If both are positive you likely pose an infectious risk. If the second test is negative, then your results are inconclusive, and you should get a PCR test to be certain.

If your PCR results don’t come back in time, Piltch-Loeb says you should stay home and isolate if you don’t want any risk. Otherwise, she recommends having a conversation with your family and friends about their risk factors and decide together whether to keep your plans. If you’re asymptomatic and getting an inconsistent result, the likelihood you pose a serious threat is low—but the overall risk is higher if your loved ones are unvaccinated or vulnerable to severe disease.

“This is not perfect science,” Piltch-Loeb says. “There’s not a great answer. You’re left with some uncertainty as to what your status is.”

5) Can I get a vaccine or booster now and be protected from Omicron? 

The bad news is that there’s only so much that getting your first shot—or even a booster dose—can do for you immediately. It takes time for the vaccine to train your immune system to protect against the virus. Studies show that two doses provide much greater protection than one—and it takes more than a month to complete a two-dose regimen of any of the vaccines.

Meanwhile, it’s not clear how effective the two-dose regimen will be against Omicron for the newly vaccinated. But new evidence suggests that the immunity generated by two doses wanes over time. That makes a booster dose particularly critical for those who received the full dose of an mRNA vaccine more than six months ago, or the J&J shot more than two months ago, in protecting against Omicron. But the effects of the booster, too, still take about two weeks to fully kick in.

Still, experts point out that some protection against the virus is still better than none. Even if you’re not protected in time for holiday gatherings, Mathema argues that you’re going to want that protection in the weeks ahead as Omicron continues to surge across the country.

“If there’s any moment to get vaccinated, this is the moment,” he says. “Because one thing this pathogen will do is it will find the unvaccinated.”

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