The Omicron variant of the coronaviruses is very fast moving. The symptoms usually appear a few days after the infection, with the viral levels peaking less than five days after the pathogen is first detected.
Even after other people in the household return to work or school, some people still have the virus, with at- home tests coming back positive day after day. Some people test positive for the virus for 10 or 12 or 14 days, but are they still infectious?
It's a great question, and I get asked it a lot, said Dr. Peter Chin-Hong, an infectious disease specialist at the University of California, San.
It is not easy to answer. Rapid antigen tests can flag people with high levels of the coronaviruses, but they are not perfect predictors of infectiousness.
Some people continue to test positive for a long time even after they stop testing because scientists don't understand why.
Experts say it's difficult to know how to interpret the results of the tests because they may pick up viral debris from a waning infection.
Some people may not be infectious at the end of their course if they are still positive for a vaccine.
Scientists disagreed on the best course of action for people who test positive for more than 10 days. Some say that the most prudent path is to continue to isolated, while others say that it's pointless for most otherwise healthy people.
Some experts advise that test results at the end of an infectious disease should be viewed as just one piece of information considered in concert with other factors, including a patient's symptoms and immune status. Dr. Chin-Hong recommended using the rapid test as a guide but not the be-all and end-all.
People with Covid-19 were most likely to spread the virus in the days before and after they developed symptoms.
The Centers for Disease Control and Prevention shortened its isolation guidelines late last year. Many people can leave isolation after five days, even though they should wear a mask through Day 10, according to the current guidelines.
There is a lot of variation in the results of rapid antigen tests.
A new analysis of people who sought repeat testing at a California site during the Omicron wave shows that 71 percent were positive for the virus four days after their symptoms appeared. According to the study, 20 percent were still positive on Day 11 despite the percentage declining over the following days.
Some people are seeing a course of being antigen-positive for a long time.
Two recent studies, neither of which has been reviewed by experts, suggest that some people with Omicron infections have the ability to replicate in a cell culture or a dish of live cells.
That is a good sign that they are likely to be infectious, according to Dr. Amy Barczak, an infectious disease expert at Massachusetts General Hospital.
In the Boston University study, a small number of participants had viable virus as late as Day 12 after their symptoms began, but most participants no longer had positive viral cultures six days after their symptoms began.
There is not a perfect correlation between a person's test results and whether their virus can be grown in culture. The Boston University researchers found that a positive test was not a good indicator of a positive culture.
The author of the study said that a positive test is not helpful.
Some people who had positive viral cultures tested positive for antigen-positive, which is slightly beyond the point of having positive viral cultures. This suggests that at the end of an infectious disease, there may be a brief period during which the tests are able to detect residual bits of the disease. She said that the study was not large enough to make conclusions about how common this would be or how long the effect would last.
Some people test positive longer than others. People with weak immune systems are more likely to take longer to fight off the virus, according to scientists.
The University of Michigan School of Public Health's infectious disease epidemiologist said that people exposed to large amounts of the virus might take longer to clear it from their systems.
Some people do a test like they barely touch their nose, while others do it in a more aggressive way.
Experts said that people who leave isolation before Day 10 should wear a mask and take precautions because of the long positive results.
Scientists disagreed. Even if it takes more than 10 days, some people recommended that people isolated until the test came back negative.
Dr. Grad said that we can now tailor recommendations to individual experiences using the rapid test results.
It doesn't make sense to ask healthy people to keep testing for more than 10 days, according to several others.
Nobody is saying that there aren't some people who might transmit after Day 10, according to Dr. Chin-Hong. He said that 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.
Experts say that there are some circumstances in which people should continue to test. They include people who are not improving and those who are immunocompromised, as they may shed infectious virus for longer periods of time. The C.D.C. says that people with weakened immune systems should be isolated for up to 20 days.
People who take the antiviral drug Paxlovid may see their symptoms rebound after stopping it.
People who have recently recovered from Covid might want to take a rapid test to make sure they are healthy before going to a large event or meeting a person with an illness. If they test positive, they should continue to live.
She said that they are probably less contagious than they were in the beginning.
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