By now, you’re probably very familiar with the guidance: If you test positive for the coronavirus, according to the Centers for Disease Control and Prevention, you should isolate for 10 days, with some options, involving testing and masking, to leave before that.
But, what happens if you keep diligently swabbing your nose … and keep getting that telltale pink line? Like this:
There isn’t a clear-cut answer here. According to the CDC, it’s very unlikely that you can pass on infectious virus particles to someone 10 days after becoming symptomatic. Researchers determine how long someone can be infectious by taking samples from someone who has tested positive for the virus, and trying to grow an infectious virus from it. According to the studies that the CDC cites, it’s pretty uncommon to be able to grow infectious virus from a sample taken after 10 days of infection. (The exception to this is for people who had severe COVID-19, or if they’re moderately or severely immunocompromised. In both of these situations, it can take longer to clear an infection because the immune system can’t get it under control.)
Those studies were done pre-omicron. But early data suggests that a maximum infection period of 10 days holds for omicron, too: a small preprint from Japan’s National Institute of Infectious Diseases found that they were still unable to isolate infectious virus from vaccinated people after 10 days. (Preprints are preliminary reports or studies that have not been peer-reviewed yet, so the data has not been vetted by the wider scientific community.)
The fact that some people can keep testing positive after day 10 sounds like it’s firmly at odds with those facts, but it’s not. Taking a rapid test has become synonymous with asking “can I spread COVID right now?,” since the tests are a useful tool to help identify if you might be infected in the first place. But it’s actually very difficult to tell from a rapid antigen test if you’re still infectious after a period of having COVID. According to Dr. K.C. Coffey, an assistant professor of epidemiology and public health and medicine at the University of Maryland School of Medicine, a positive test could be detecting viral antigens that have already been bound by antibodies, which would make them nontransmissible. (It’s like having them try to run a 100-meter dash while dressed like the Stay-Puft Marshmallow Man—they’re not going to get very far, and if they hit something, they’ll just bounce off. In this metaphor, it is your immune system that has forced them to dress up like the Stay-Puft man.) So: While a positive result after 10 days could, in theory, mean that the virus is still replicating in your body, according to Coffey, it’s by no means firm evidence that it is.
What to do in the face of that uncertainty? The two experts I checked with, Coffey, as well as Dr. Robin Colgrove, an assistant professor and infectious disease specialist at Harvard Medical School, agreed that after 10 days of isolation, you probably don’t need to keep testing at all if you’re not immunocompromised. This is in line with the CDC guidelines. While zealously doing additional tests may seem prudent, it doesn’t stand to be that useful. “You should never do a test unless you are prepared to deal with the results of the test and you know how to interpret a positive or negative,” Colgrove told me. “If you don’t, then there’s no point in doing the test. It just causes trouble.”
There are two exceptions. One is if you’re immunocompromised, because, as previously mentioned, it could take longer than 10 days for your body to clear an infection, or you might need extra help to do that. (Yes, it is frustrating that there is no clear, easy-to-access guidance on what you should do if this is your situation.) Additionally, if you’re caring for an immunocompromised person, you probably want to test negative first. The worry is that even a small amount of infectious virus could spell big trouble for a patient with a compromised immune system.
If you keep testing nonetheless, and your rapid test does turn up positive after 10 days, Coffey advises that you take additional precautions, like wearing a mask, keeping up good hand hygiene, and avoiding people who are high-risk (including those who are immunocompromised) until 14 to 20 days after infection and abatement of symptoms, just in case.