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. 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 a 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 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,
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.
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 Garner, a clinical microbiologist at the University of California, Los Angeles, Health System
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