Seeing that bright red line appear on a home Covid-19 test can feel inevitable during a surge like the present. What can surprise is how many days later this line keeps appearing.
The Centers for Disease Control and Prevention recommends five days from the onset of symptoms or diagnosis as the minimum isolation period before infected people can return to public activities while taking certain precautions. Still, some people continue to test positive for the Covid-19 virus on rapid tests beyond those five days. Some even test positive after 10 days and after symptoms have subsided.
The question then becomes: How long are people contagious? In the third year of the pandemic, as new, more contagious variants remain prevalent and more people have built up immunity, researchers aren’t exactly sure when or how long people with Covid-19 might be contagious and shedding viruses, particularly on that end of an infection.
“That’s still up for debate,” said Nathaniel Hafer, operations manager at the University of Massachusetts’ Center for Clinical and Translational Science.
The isolation guidelines the CDC has updated in light of Omicron’s rapid spread are likely to result in some people exiting isolation while contagious, some health experts and clinicians said, especially if people aren’t waiting for a negative rapid test and not high ones Wearing shoes. Quality masks for a full 10 days.
“We know that people will return to work while they’re still contagious, even if they’re feeling better,” said Carina Marquez, associate professor and physician of infectious diseases at the University of California, San Francisco.
dr Marquez said she recently tested positive for an at-home test for 13 days, a time when she chose to work remotely. For those without that option, Dr. Marquez that they should stop isolating after a negative rapid test, or they should stop isolating after 10 days and wear a mask, whichever comes first.
The CDC recommends that everyone wear a well-fitting mask and avoid traveling and being around high-risk individuals for 10 days, regardless of when the person leaves isolation.
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The CDC did not immediately comment on its guidance.
No test can accurately tell whether someone is contagious with Covid-19, infectious disease experts and laboratory technicians have said. A more accurate indicator is culturable virus, a test of whether a sample of virus taken from a patient can infect cells in a laboratory. However, such tests are complex and can only be performed in laboratories with specific safety protocols.
Several studies suggest that positive results on rapid antigen tests such as iHealth, QuickVue, and BinaxNow often coincide with culturable virus results. Some public health experts have encouraged people to use home tests as a measure of infectivity, especially since they have become more widely available.
The tests often only turn out positive when a person is carrying large amounts of the virus. At the beginning of an infection, when a person’s viral load increases, it can take a few days before the tests come back positive. For this reason, health authorities recommend that people with symptoms and negative rapid test results wait and retest or get a more sensitive laboratory-based PCR test.
When a person’s viral load drops, rapid tests are a better indicator of who is no longer contagious, public health experts said. The University of Chicago began conducting the testing in December to determine which healthcare workers can safely return to work and alleviate staff shortages from 10 days of isolation, said Emily Landon, executive medical director for infection prevention and control at UChicago Medicine.
More than 40% of 260 healthcare workers who felt well enough to work tested positive when they first took a rapid test between days five and 10 of an infection, Drs. Landon and her colleagues found that it is not peer-reviewed, according to a preprint study.
“We didn’t feel comfortable when they came back,” said Dr. landon
According to the CDC, the data show that most transmissions occur early in the course of infection and that peak infectivity decreases within a week of onset of symptoms. Much of this data was collected before an Omicron variant emerged that was far more contagious than its predecessors.
Recent studies have shown that a significant proportion of people test positive on rapid tests after five days, including a study published by the CDC in February that found that 54% of people die between days five and ninth after symptoms or had positive rapid test results at the time of diagnosis. The percentage of positive results decreased over time. The authors wrote that the evidence reinforces the importance of mask use during this time and that rapid testing could be a useful tool in making isolation recommendations.
A recent study found that only 17% of vaccinated college students had culturable virus beyond day five, while a separate study of people with mild Covid-19 cases found that more than 50% by day five and 25% had culturable virus on day eight. None of the studies were peer-reviewed.
“It worries us that by day five or eight, people are still capable of transmission,” said Amy Barczak, an infectious disease physician at Massachusetts General Hospital in Boston and senior author of one of the studies.
Both studies found that a negative rapid test after an initial diagnosis is a good indicator that a person no longer has the culturable virus and a solid sign that they are unlikely to be contagious anymore. However, if a rapid test result is positive towards the end of the 10 days, it can be more difficult to tell how contagious that person might be, especially if they are feeling better and the line is fading.
“You should assume that you may have a transmissible virus,” said Tara Bouton, an assistant professor of medicine at Boston University School of Medicine and lead author of another study. People who stop isolating before 10 days should wear a quality mask like an N95 and keep contacts to a minimum, said Dr. button.
Immunocompromised people and those who become seriously ill can be contagious even longer, studies have shown, and patients who show symptoms again or test positive again after taking Pfizer inc
Paxlovid pills should also be considered contagious, infectious disease experts said.
In general, studies indicate that the virus is rarely culturable after 10 days. Infectious disease experts are divided on whether most people should continue testing past this point.
“These tests are not perfect,” said Dr. Landon of UChicago. “They underestimate the contagiousness at the beginning of an illness and overestimate it at the end … but not so much.”
write to Brianna Abbott at [email protected]
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