Here's what we know so far about when to take a rapid test if you're vaccinated against COVID
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.
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.
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.
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.There is no harm in testing two days after exposure.
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.
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