By Rae Ellen Bichell
Monday, Jan. 31, 2022 (Kaiser News) — What does it mean if someone’s rapid antigen test result comes back positive after five days of covid-19 isolation? According to the experts, that person most likely still has a viral load high enough to infect others.
“Any time you’re positive on one of these rapid at-home covid tests, it means you still have a really high level of the viral protein, and most experts interpret that as a high level of virus in your nose. passage,” said Matthew Binnicker, director of clinical virology at the Mayo Clinic and president of the Pan American Society for Clinical Virology.
The Biden administration is buying a billion rapid antigen tests to ship to US homes, aiming to address the need for more Covid testing and reduce a nationwide shortage of testing. That influx of tests comes on the heels of revised federal guidelines that shortened the isolation period to five days from the onset of symptoms or a positive test followed by five days of masking.
It’s well known that PCR tests, the highly sensitive polymerase chain reaction tests that detect viral genetic material, can pick up “dead” virus — material that can’t replicate — and give a positive result weeks after recovery, even when the person being tested isn’t. more contagious.
But virologists say this is unlikely to be the case for the rapid antigen tests — which detect certain proteins in the virus and provide results within 30 minutes — that are making their way into people’s mailboxes.
The odds are very slim, outside of a lab setting, that someone who gets a positive result on a rapid antigen test is a noninfectious person who is shedding large amounts of dead virus, said Kelly Wroblewski, director of infectious disease programs for the Association of Public Health Laboratories.
There are several reasons for this. First, according to the Infectious Diseases Society of America, a “big advantage” of rapid antigen testing is that they are somewhat blunt instruments for picking up the virus. Therefore, a positive result usually means the person has a high viral load, especially if symptoms appear.
Second, it takes a significant amount of virus material to register as positive on an antigen test, so a person would need a high level of dead virus in their nose or throat, and usually that indicates a high level of live virus too. .
Third, even if someone had a high level of dead virus nearby, the amount alone would not be enough to cause a positive test – viral proteins tend to lose their shape after a battle with the immune system, causing they may become undetectable by antigen testing.
In a lab setting, a rapid antigen test could detect bits of “dead” virus, said Dr. Sam Dominguez, an infectious disease pediatrician and medical director of the clinical microbiology lab at Children’s Hospital Colorado.
Federally funded scientists did just that to check whether rapid antigen testing would work just as well on omicron as on other covid variants. Dominguez said, however, that the chances of dead virus — and only dead virus — causing a positive antigen test in a human rather than in a petri dish, are low.
Unlike scientists in a lab, the human body doesn’t just try to inactivate viruses in attack mode. It tries to erase them, proteins and all. “RNA can last longer than the proteins,” Wroblewski said, but neither substance is known for its sturdiness. Even for scientists, keeping proteins intact after inactivating the virus is an art.
“It’s not easy,” said Dr. Izabela Ragan, a veterinary scientist at Colorado State University who has a focus on human virology and who spent about two years developing a vaccine against SARS-CoV-2. Her work revolves around killing the virus while preserving the shape of its proteins.
The confusion over how to interpret test results stems largely from this conundrum: While a wealth of testing options are available, there is no surefire way to measure whether a person is actually contagious.
“I wish there were,” Binnicker said. “There is no test we have for infectiousness.”
The best option is to take a patient’s sample, put it in a dish of thriving living cells, and see what happens to the healthy cells. If there is a live virus, said Dirk Dittmer, a virologist at the University of North Carolina-Chapel Hill, “then the cells would die, just like the lung cells in your body would die.” But the finicky process takes about three days and can only be done in labs approved to deal with such pathogens, making it impractical.
Instead, the most widely available tests rely on completely different methods. They fall into two categories, usually called molecular tests and antigen tests.
Molecular tests, including PCR, look for specific pieces of RNA from the virus. With the exception of some over-the-counter options, the tests are processed in a lab. They are able to detect tiny bits and pieces of the virus’s genetic material by copying everything floating around in a person’s sample over and over, amplifying it “a million to a trillion times,” said Dr. Marie Louise Landry, director of the Clinical Virology Laboratory at Yale New Haven Hospital. That’s why, she explained, “PCR can detect very low levels of viral RNA for weeks and even months after infection, when a patient is no longer infectious.”
Rapid antigen tests, on the other hand, look for viral proteins rather than the genetic material of the virus. Many of them look for nucleocapsid or “N” proteins, which are abundant in infected cells and form a protective capsule around the virus’ genetic material. Unlike molecular tests, rapid antigen tests only work with what is available – there is no copying or amplification involved. These tests drag a person’s sample across a special piece of paper that contains a fence of antibodies designed to grab the virus’ N proteins. If enough proteins catch on the fence, a visible color line will appear.
“Even single molecules can make a PCR positive,” says Dr. Michael Mina, an epidemiologist who has been an outspoken advocate for rapid testing during the pandemic and is now Chief Science Officer for healthcare company eMed. But, he added, it takes “about 100,000-1 million molecules to make the rapid antigen test positive.”
But what about the official recommendation that people with Covid should isolate at home for five days – followed by five days of masking? Data from the sports world, where resources and testing methods abound, offer a glimpse of just how variable that period of infectivity can be.
A preliminary study of NBA players and staff found that of the 70 people infected with ommicron, on day 5 after their first positive test, about 40% of them were still likely to be contagious.
“What we recommend is that you can do an antigen test between days 5 and 10. If it’s positive, you’ll be in isolation until day 10,” Binnicker said.
CDC public affairs specialist Jasmine Reed said the best approach is to use an antigen test toward the end of the five-day isolation period if the person’s symptoms have improved. “If your test result is positive, you should continue isolating until day 10,” she said.
Wroblewski said an important measure of infectiousness is the simplest: how a person is feeling.
“If you have a high fever and cough, don’t approach people. I feel like we somehow forgot that part,” she said. “Let’s not put so much emphasis on the test and the technology that we forget the basic infection control practices: that if you’re sick, stay home.”
KHN (Kaiser Health News) is a national newsroom that produces in-depth journalism on health issues. Together with Policy Analysis and Polling, KHN is one of the three major operational programs of KFF (Kaiser Family Foundation). KFF is an endowed non-profit organization that provides information on health issues to the nation.
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