In the lead-up to Thanksgiving, Americans are no stranger to planning. But this year, as they prepare to let turkeys brine and pie crusts thaw, people across the country are waiting for something extra: a coronavirus test they hope can clear them to mingle with loved ones.
Many people consider a negative coronavirus test to be a ticket to freely socialize without precautions. But scientists and doctors say this is dangerously misguided. It is one precautionary measure but does not negate the need for others, like quarantining, masking and distancing.
The main reason is that a test gives information about the level of the virus at one point in time. A person could be infected but not have enough virus yet for it to register on a test. Or, a person may become infected in the hours or days after taking a test. Also, the tests do not have 100 percent accuracy.
“If you require all of your guests to email you a negative test result before your Thanksgiving dinner, it will definitely decrease the risk of an outbreak — but not completely,” said Dr. KJ Seung, chief of strategy and policy for the Covid response at Partners in Health. Yet this is a common misperception contact tracers hear when talking to people, he said.
The experts agreed that tests were very useful for one thing: If someone receives a positive test, that person knows to stay home and isolate. But a negative test, while helpful, is not sufficient, said Dr. Esther Choo, an emergency medicine physician and a professor at Oregon Health and Science University.
A test “filters out those who are positive and definitely shouldn’t be there,” she said. “Testing negative basically changes nothing about behavior. It still means wear a mask, distance, avoid indoors if you can.”
Not all tests are created equal
Different tests for the coronavirus give different information.
Laboratory tests that rely on a technique called polymerase chain reaction, or P.C.R., can detect the virus when it’s present even at very low levels. But it might take a couple of days to return results, leaving time for someone to be exposed. Antigen tests are faster, less expensive and more convenient — they can deliver results in a matter of minutes — but are also more prone to missing the virus when it’s scarce. To receive emergency authorization from the Food and Drug Administration, antigen tests for the coronavirus need to detect only 80 percent of the infections found by P.C.R. Many rapid tests also aren’t authorized for use in people who don’t have symptoms.
In some cases, a person who tests negative with an antigen test might test positive by P.C.R. — raising the risk that a negative antigen test could give someone a false sense of security en route to Thanksgiving dinner, said Paige Larkin, a clinical microbiologist at NorthShore University HealthSystem in Chicago, where she specializes in infectious disease diagnostics.
Why tests alone aren’t enough
“A negative result is a snapshot in time,” Dr. Larkin said. “It’s telling you that, at that exact second you are tested, the virus was not detected. It does not mean you’re not infected.”
After infiltrating a person’s body, the virus can take several days to build up, and in the meantime, there may not be enough virus for a test to detect it. But the person could still be infected or contagious in the interim. A person who tests negative one day might turn positive just a day later, or even an hour later. People can spread the virus in the days before they first start feeling ill, and can also spread the virus even if they never develop symptoms.
“The challenge then for the individual is that a negative test today does not mean that a person will be negative tomorrow or the day after that,” said Natalie Dean, an assistant professor of biostatistics at the University of Florida.
Taking multiple tests over a period of days gives a clearer answer. But experts cautioned that no test — regardless of how many times it’s taken in succession — can definitively determine whether someone infected by the coronavirus is contagious, or no longer poses a transmission risk to other people.
The October outbreak at the White House is a good example of what can happen when a group of people rely heavily on testing and ignore other strategies to limit the spread of the virus.
In an informal survey of 670 epidemiologists, just 6 percent said that if they recently tested negative for the virus, they would be comfortable spending time indoors with others without precautions. Twenty-nine percent said they would do so if everyone also used masks and stayed distant. And a full 64 percent said that even if they tested negative, they would not be comfortable spending time indoors with people they don’t live with.
The Coronavirus Outbreak ›
Words to Know About Testing
Confused by the terms about coronavirus testing? Let us help:
- Antibody: A protein produced by the immune system that can recognize and attach precisely to specific kinds of viruses, bacteria, or other invaders.
- Antibody test/serology test: A test that detects antibodies specific to the coronavirus. Antibodies begin to appear in the blood about a week after the coronavirus has infected the body. Because antibodies take so long to develop, an antibody test can’t reliably diagnose an ongoing infection. But it can identify people who have been exposed to the coronavirus in the past.
- Antigen test: This test detects bits of coronavirus proteins called antigens. Antigen tests are fast, taking as little as five minutes, but are less accurate than tests that detect genetic material from the virus.
- Coronavirus: Any virus that belongs to the Orthocoronavirinae family of viruses. The coronavirus that causes Covid-19 is known as SARS-CoV-2.
- Covid-19: The disease caused by the new coronavirus. The name is short for coronavirus disease 2019.
- Isolation and quarantine: Isolation is the separation of people who know they are sick with a contagious disease from those who are not sick. Quarantine refers to restricting the movement of people who have been exposed to a virus.
- Nasopharyngeal swab: A long, flexible stick, tipped with a soft swab, that is inserted deep into the nose to get samples from the space where the nasal cavity meets the throat. Samples for coronavirus tests can also be collected with swabs that do not go as deep into the nose — sometimes called nasal swabs — or oral or throat swabs.
- Polymerase Chain Reaction (PCR): Scientists use PCR to make millions of copies of genetic material in a sample. Tests that use PCR enable researchers to detect the coronavirus even when it is scarce.
- Viral load: The amount of virus in a person’s body. In people infected by the coronavirus, the viral load may peak before they start to show symptoms, if symptoms appear at all.
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