As the Omicron variant continues to spread wildly, millions of people are worried they have been exposed—and are desperate to find out if they are infected. But in many areas, at-home rapid antigen tests are difficult to find, and PCR tests—if an appointment is available—can take days to return a result. So if you can get your hands on a precious test, when is the best time to use it for the most accurate result?

With the original strain of SARS-CoV-2, the coronavirus that causes COVID, the amount of virus in the body—known as viral load—generally reaches its peak five to seven days after exposure, says Gigi Kwik Gronvall, an immunologist at the Johns Hopkins Bloomberg School of Public Health. The Centers for Disease Control and Prevention and many state health departments recommend that exposed people who have been vaccinated wait until this time to take a test after contact with someone who has COVID to be certain that a negative test is truly negative because the highest viral load will provide the best chance of detecting the virus.

But with the Omicron variant, that may no longer be the case: symptoms seem to appear sooner after infection. As a result, infected people could spread the virus to others sooner as well. Recent studies suggest that people infected with Omicron may continue to shed the virus for many days after the onset of symptoms, meaning that the current CDC recommendation to isolate for five days after infection may not be long enough.

What can different test types tell you?

The two main types of COVID tests available to consumers are vastly different in their sensitivity. A PCR test, which measures viral RNA in the body, can detect as few as 200 copies of SARS-CoV-2 per milliliter of sample. A rapid antigen test, by contrast, may not come back positive if there are fewer than 500,000 viral copies.

It is unclear whether a higher viral load necessarily means a person is more infectious—after all, the amount of virus replicating in the body does not necessarily track with the number of viral particles that person emits when they breathe. But Gronvall says that the assumption is that a positive rapid antigen test is a good marker for contagiousness because these tests do not detect SARS-CoV-2 until peak viral load. A PCR test is so sensitive that it may continue to show the virus in the body long after a person is no longer contagious.

At this point in the pandemic, it has become more difficult for epidemiologists to say with certainty whether one variant reaches a higher viral load or how that viral load correlates with infectiousness, notes Ajay Sethi, an epidemiologist at the University of Wisconsin–Madison. That is because so many people have now been infected with COVID or received different numbers of vaccine doses, meaning that their immune systems respond differently to the newer variant. “It’s too complicated to say one variant will produce a higher viral load,” Sethi says.

What is the science behind the five-to-seven-day wait recommended by the CDC and many state health departments?

The guidance is based on the average time between infection and emergence of symptoms with the original strain of SARS-CoV-2: If symptoms do not emerge after five days, there is a very low chance that a person is infectious. The theory is that viral load and symptoms peak around the same time, providing enough virus for a PCR or rapid antigen test to definitively give a positive or negative result.

But the Omicron variant’s symptoms seem to come much earlier: three days after exposure on average. Furthermore, emerging evidence suggests that people with Omicron can become contagious at least a day before symptoms appear. A January 5 preprint paper posted on medRxiv tracked 30 people during an Omicron outbreak and found that most were infectious several days before rapid antigen tests detected COVID.

Further confusing matters is a recent study of patients in Japan that found the Omicron variant might not reach peak viral load until three to six days after the onset of symptoms. But that does not mean people are not infectious much earlier, says Omai Garner, a clinical microbiologist at the University of California, Los Angeles, Health System.

Because PCR tests can detect an infection before viral load peaks, they might be better at spotting Omicron right after a person acquires the variant but before they can spread the infection. Right now, however, it can take several days for PCR tests to return results, by which point the person will be capable of infecting others.

If people with Omicron become infectious two days after exposure, why should we wait to get tested?

There is no harm in testing two days after exposure. But the concern is that a negative antigen test at two days could provide a false sense of security because the viral load may still be too low at this point to be detected by rapid antigen tests.

A negative test one or two days after a person is exposed—particularly a rapid antigen test—will be meaningless, and that person will need to test again before coming out of isolation. “They can’t assume they don’t have COVID,” Garner says. “They need to act like they have COVID until they get a result that can affirm that they don’t.”

The current shortage of tests complicates matters because it could discourage people from seeking such additional testing. “It speaks to a lack of federal response to this virus,” Garner says. Every time cases surge, “we can’t get enough testing.”

How long do people remain infectious?

Data on the original coronavirus strain have shown that most people with mild COVID are no longer infectious after 10 days following symptoms. Garner says that the virus may act differently in people who are vaccinated, unvaccinated or only recently vaccinated, but there is little information on those differences at the moment.

On December 27, 2021, the CDC changed its isolation recommendations to suggest that people with mild symptoms of COVID isolate for five days and then return to work with a mask, even if they have not received a negative test. The agency suggested that this guidance was based on new science, although it has not released the data, and many anecdotal cases suggest that Omicron infection can last longer than five days.

And many experts believe that eliminating the testing requirement has more to do with avoiding test shortages than it does with new science, especially because so little is still known about Omicron. “We put all this effort into discovering infectious people... why not at LEAST ensure those we do discover don’t spread on,” wrote infectious disease expert Michael Mina in a Twitter thread in which he called the CDC decision “reckless.”

Are better tests on the horizon, including those that use saliva collected from the mouth or throat?

Initial evidence suggests that Omicron may be more prevalent in the throat than previous variants, which tend to replicate in the nose. Small initial studies have found that both PCR and antigen tests using saliva swabs produce positive results days earlier than nasal swabs. Numerous anecdotal reports also suggest that throat swabs produce more accurate results, although one study suggests that rapid antigen tests are less sensitive when used in the throat instead of the nose, as is recommended.

Although a few experts have advocated for the collection of saliva, the Food and Drug Administration recommends against at-home throat swabs: In a January 7 Twitter thread, the agency said that, in addition to safety concerns, “we don’t have any data yet suggesting throat swabs are an accurate or appropriate method for at-home tests.” Changing this guidance would require more research and the standardization of new types of tests, Garner says.

Bottom line: When should a person take a COVID test?

Taking a PCR test at three days can tell you whether you have the virus, even if you are not contagious yet.

If PCR tests are not available, and you only have access to one antigen test, your timing largely depends on the situation. If you have the ability to stay isolated and know exactly when you were exposed, Sethi and Garner agree that waiting five days to take an antigen test is your best bet to ensure that a negative test is actually negative or whether the symptoms you are experiencing are caused by COVID.

But if you are asymptomatic, need to be in contact with people and have only one rapid antigen test, taking it at two days might be worthwhile. If it is positive, Sethi says, you can assume that you have COVID. “If it’s negative, you’re going to wish you had another test,” he adds.

Either way, experts say you should act as if you have COVID until enough time has passed to know that the test is truly negative. A negative test—particularly a rapid antigen test—does not mean you are not infectious or will not become so later on. “If I’ve said one thing over and over, it’s that testing is a moment in time,” Gronvall says.

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