June 1, 2022 -- As Omicron subvariants continue to spark an increase in COVID cases across the U.S., some people are finding that they are testing positive for long periods of time.

At-home antigen tests may return positive results for 10 days -- or even longer, up to 14 days, according to The New York Times. Public health experts have said it’s been difficult to understand what that means in terms of contagiousness since rapid tests can’t always predict that with accuracy.

“Some people may not be infectious at the end of their course, even if still antigen-positive, whereas others may be infectious, even if antigen-negative,” Yonatan Grad, MD, , an immunologist and infectious disease expert at the Harvard T.H. Chan School of Public Health, told the newspaper.

Rapid antigen tests detect proteins on the outside of the coronavirus, which can accurately pinpoint people who carry high levels of the virus. Studies have suggested that most people stop testing positive on these tests within the first 10 days of infection.

But a growing group of people seem to be testing positive for longer. This could mean that some people may be shedding viral particles for a longer period or that the tests are picking up leftover viral debris as their infection fades, the Times reported.

A few recent preprint studies, which haven’t yet been peer-reviewed, have indicated that some people who get the Omicron variant may test positive for longer and spread the virus for longer than previous strains.

In one analysis of people who were tested in California during the Omicron wave in January, about 65% of people who retested were positive 5 days after their symptoms began or after their first positive test. That dropped in the following days for most people, but about 20% still tested positive on day 11.

In another recent study, some people with Omicron infections in Massachusetts shed viral particles for more than a week. About 25% still had viable virus particles on day 8 or later.

In yet another study, which included vaccinated students and staff at Boston University, most participants no longer had positive viral cultures about 6 days after their symptoms began. But 17% had positive cultures after that, with the latest up to day 12.

The tricky aspect is that a negative antigen test appears to be a reliable indicator for negative viral cultures in a lab, but a positive test doesn’t necessarily predict positive viral cultures, the researchers found.

“You can be somewhat reassured by a negative test, but the positive test is not particularly helpful,” Tara Bouton, MD, the lead study author and an infectious disease specialist at the Boston University School of Medicine, told the newspaper.

Now researchers are trying to understand why some people test positive for longer periods of time. People with weaker immune systems tend to take longer to clear the virus, though some healthy and fully vaccinated people may continue to test positive. What’s more, those who had higher exposures to the coronavirus may take longer to clear it, some tests are more sensitive than others, and people likely swab their noses differently, the newspaper reported.

Due to the uncertainty, scientists are split on what people should do when they test positive for more than 10 days. Some have advocated for ongoing self-isolation to prevent potential transmission, and others have said that prolonged isolation is unnecessary.

People who continue to have symptoms or are immunocompromised, for instance, may continue to shed viral particles for longer periods of time and should continue to isolate, the Times reported. Generally, healthy people who have recovered can end their isolation after 10 days but should continue to wear a well-fitting mask in public.

Overall, tests should be used as one factor at the end of an infection, along with a patient’s symptoms and immune status. That means “using the rapid tests as a guide but not the be-all and end-all,” Peter Chin-Hong, MD, an infectious disease specialist at the University of California at San Francisco, told the newspaper.

The New York Times: “Can I Stop Isolating If I’m Still Testing Positive for the Virus?”

medRxiv: “COVID-19 symptoms and duration of direct antigen test positivity at a community testing and surveillance site, January 2021-22,” “Duration of viable virus shedding in SARS-CoV-2 omicron variant infection,” “Viral dynamics of Omicron and Delta SARS-CoV-2 variants with implications for timing of release from isolation: a longitudinal cohort study.”

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