The Omicron variant of the coronavirus moves fast. Symptoms typically appear just a few days after infection, with viral levels peaking less than five days after the pathogen first becomes detectable.
But for some people, the virus seems to linger, with at-home tests coming back positive day after day, even after other people in the household return to work or school. So why do some people test positive for the virus for 10 or 12 or even 14 days — and are they still infectious after so long?
“It’s a great question — it’s one that I get asked all the time,” said Dr. Peter Chin-Hong, an infectious disease specialist at the University of California, San Francisco.
It is also one without an easy answer. Although rapid antigen tests, which detect proteins on the outside of the coronavirus, can flag people carrying high levels of the virus, they are not perfect predictors of infectiousness.
Studies suggest that while most people stop testing positive on antigen tests sometime during the first 10 days of their illnesses, a notable subset of people continue to test positive for longer, for reasons that scientists do not entirely understand.
In some cases, these people may still be shedding infectious virus, but in others, the tests may be picking up viral debris from a waning infection, experts say, making it difficult to know how to interpret the results.
“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,” said Dr. Yonatan Grad, an immunologist and infectious disease expert at the Harvard T.H. Chan School of Public Health.
Indeed, scientists disagreed on the best course of action for people who test positive for more than 10 days. While some say that the most prudent path is to continue to isolate, others argue that prolonged isolation is unnecessary for most otherwise healthy people.
Given the uncertainty, some experts have advised that test results at the end of an infection be viewed as just one potentially useful piece of information considered in concert with other factors, including a patient’s symptoms and immune status. Along those lines, Dr. Chin-Hong recommended “using the rapid test as a guide but not the be-all and end-all.”
What we know
Studies conducted before the emergence of Omicron demonstrated that people with Covid-19 were most likely to spread the virus in the few days before and after developing symptoms.
The Centers for Disease Control and Prevention cited these findings when explaining its decision to shorten its isolation guidelines late last year. While the old guidelines recommended that patients isolate for 10 days, the current recommendations say that many people can leave isolation after five days, although they should wear a mask through Day 10.
Research also suggests that rapid antigen tests are most likely to be positive early in the course of illness, but there is considerable variation.
According to a new analysis of people who sought repeat testing at a California site during the Omicron wave, an estimated 71 percent were antigen-positive four days after their symptoms appeared or after they first tested positive for the virus. That percentage declined over the following days, but an estimated 20 percent were still positive on Day 11, according to the study, which has not yet been published in a scientific journal.
“For some people, they’re seeing fairly prolonged courses of being antigen-positive,” Dr. Grad said. “I think we chalk it up to some variation in people’s immune system and ability to respond to infection and clear this virus.”
Indeed, a pair of recent studies, neither of which has been reviewed by experts, suggest that some people with Omicron infections shed infectious virus — capable of replicating in a “cell culture,” or a dish of live cells in the laboratory — for more than a week.
“That’s a pretty good indication that they’re likely to be infectious,” said Dr. Amy Barczak, an infectious disease expert at Massachusetts General Hospital, who found that 25 percent of people still had viable virus on Day Eight or beyond.
In the other study, which enrolled vaccinated students and staff at Boston University, researchers found that while most participants no longer had positive viral cultures six days after their symptoms began, a small number had viable virus as late as Day 12.
What we don’t know
However, there is not a perfect correlation between someone’s antigen test results and whether their virus can be grown in culture. In a small, preliminary subanalysis, the Boston University researchers found that while a negative antigen test was a reliable indicator that the person would also have negative viral cultures, a positive test was not predictive of a positive culture.
“You can be somewhat reassured by a negative test, but the positive test is not particularly helpful,” said Dr. Tara Bouton, an infectious disease specialist at the Boston University School of Medicine and an author of the study.
Dr. Barczak’s team found that some people tested antigen-positive slightly beyond the point of having positive viral cultures. This suggests that at the end of an infection, there may be a brief period during which the tests are simply detecting lingering bits of viral protein. The study was not large enough to draw conclusions about how common this would be or how long the effect might last, she said.
Precisely why some people test positive longer than others is not entirely known. In general, people with weaker immune systems are likely to take longer to fight off the virus, scientists said, although even young, healthy and fully vaccinated people may be positive for extended periods.
Another possibility is that people exposed to large doses of the virus might take longer to clear it from their systems, said Aubree Gordon, an infectious disease epidemiologist at the University of Michigan School of Public Health.
Tests also differ in their sensitivity, and people differ in how they perform them. “Some people do a swab test and it’s like they barely touch their nose,” Dr. Gordon said. “Whereas with other individuals — actually, with a family member recently, I had to be like, ‘Don’t hurt yourself,’ because they were really, really getting in there.”
What to do
One lesson is that prolonged positive results are common enough that people who leave isolation before Day 10 should continue to take precautions, such as wearing a well-fitting mask, experts said.
Beyond that, scientists disagreed. Some recommended that people isolate until they test antigen-negative, even if it takes more than 10 days.
“We can now tailor recommendations to individual experiences using the rapid test results to guide us,” Dr. Grad said. “And since we know that some people can have prolonged courses, it seems reasonable to me that if you are able to continue isolating if positive, you should do so.”
But several others said that, as a matter of public health policy, it does not make sense to ask most otherwise healthy people to isolate, or even keep testing, for more than 10 days.
“Nobody’s saying that there aren’t some people, maybe statistically speaking at the end of the tail, who might transmit after Day 10,” Dr. Chin-Hong said. But people at that stage of infection would not be likely to play a large role in spreading the virus, and continued testing could keep many people out of work or school without much public health benefit, he said. “And also you raise an equity issue,” he added, “like, ‘Who on Earth can have enough tests?’”
Even then, experts say, there are some circumstances in which people should continue to test and potentially isolate beyond Day 10. They include people whose symptoms are not improving and those who are immunocompromised, as they may shed infectious virus for longer periods of time. (The C.D.C. recommends that people with weakened immune systems isolate for up to 20 days.)
There are also recent reports that people who take the antiviral drug Paxlovid may see their symptoms rebound after stopping the medication. “If they have symptoms that come back again after treatment, then it might be reasonable to extend isolation and to think about using tests,” Dr. Bouton said. “Viral load may come up in that situation.”
And people who have recently recovered from Covid might want to take a rapid test as a precaution before engaging in any particularly risky activities, such as meeting with an immunocompromised person or attending a large, indoor event. If they test positive, they should proceed as though they might be contagious, Dr. Gordon said.
“They’re probably less contagious than they were in the in the first few days,” she added. “But I would still certainly advise some caution.”
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