A Blog by Jonathan Low


Feb 17, 2021

Why People Should Assume They're Contagious

The data reveal that false positives in Covid tests are rare and that 20% of those who spread the disease are asymptomatic. 

So until a majority of the population is vaccinated, it is prudent for people to assume they are contagious until they otherwise have proof.  JL

Melinda Moyer comments in the New York Times:

All the data we have so far show that when tests for the coronavirus come back positive, they are rarely wrong. “The rate of false positives is much lower than 1%.” But the test is so sensitive that it can detect viral particles that are no longer able to cause infection. So it will not tell you if you are contagious. (And) one in five people who develop Covid-19 never have symptoms.

A friend recently announced on social media that her husband had tested positive for the coronavirus. He had no symptoms, and the rest of his family tested negative. Almost immediately, people began questioning his test results in the comments. “I think it’s a false positive!” someone wrote. Two others suggested the same; one said she was sure she’d recently had a false positive coronavirus test result, too, because she hadn’t had symptoms and everyone else in her household tested negative. A scientist I know told me about an acquaintance who tested positive for the virus in January, decided his test was a false positive and did not isolate from family members who were visiting to meet his new baby.

Just to be clear: “It is very ill advised to ignore a positive result just because you don’t have symptoms,” Daniel Diekema, the director of the division of infectious diseases at the University of Iowa Carver College of Medicine, told me. “It puts others at risk.” An estimated one in five people who develop Covid-19 never have symptoms, and they can still infect others.

More important, all the data we have so far show that when tests for the coronavirus come back positive, they are rarely wrong. A large majority of cases in the United States are found with a test that uses a technology called polymerase chain reaction, or P.C.R. These tests should come back positive only if they detect genetic material that is specific to the coronavirus — which makes them far more accurate than many other kinds of diagnostic and screening tests, such as one for prostate cancer that assesses blood levels of a protein that can increase for many reasons other than malignancy.

In most cases, “the rate of false positives is extremely low ­— much lower than 1 percent,” said Ilan Schwartz, an infectious disease clinician and researcher at the University of Alberta. Many of the commonly used P.C.R. tests in the United States had no false positives when tested in labs.

But the test is so sensitive that it can detect viral particles that are no longer able to cause infection. So it will not tell you if you are contagious. People who get a positive coronavirus test should act as if they can infect others and follow public health department guidelines for isolation, unless they are advised otherwise by a health care provider or local health department.

There are situations in which false positives occur. In August, 77 N.F.L. players’ and staff members’ P.C.R. tests returned false positive results because the samples were contaminated, and in December, several coaches on Kansas City’s football team tested positive because of a computer error. Mistakes like these are typically discovered quickly because they affect many samples.

False positives are also uncommon among antigen tests, a less frequently used tool that is generally less expensive than P.C.R. tests and often returns results far more quickly. Antigen tests are known to be less sensitive than P.C.R. tests, and they don’t always detect the presence of the coronavirus in the early days of an infection. As a result, they produce more false negatives than P.C.R. tests. People who test negative on an antigen test after being exposed to the coronavirus should follow up with a P.C.R. test to confirm they aren’t infected.

However, antigen tests are very effective at identifying the presence of the coronavirus when people are most contagious, and they rarely come back positive when they shouldn’t. “The overwhelming majority of those positives will be real positives,” said Gigi Kwik Gronvall, a senior scholar at the Johns Hopkins Center for Health Security who studies coronavirus testing.

Here’s a confusing thing about P.C.R. and antigen tests: They can return more false positive results than expected when administered to people in communities with very low rates of coronavirus infection. That’s because — stay with me here — although the false positive rate is always the same, when there’s less virus circulating in a community, the number of uninfected people who could get false positive results increases. There’s a larger pool of uninfected people who would receive incorrect results. But because the United States is recording so many new cases of Covid every day, this isn’t an issue right now, Dr. Gronvall said.

Some people who distrust their positive coronavirus test follow it up with an antibody test, which is a blood test (not a nasal swab or saliva test) that can indicate whether a person was infected with the coronavirus in the past. But antibody test results can be less accurate than P.C.R. test results — so if a P.C.R. test comes back positive but an antibody test later comes back negative, it’s wise to trust the P.C.R. results.

Why do some believe, then, that false positives are common? For one, some of the messaging on test accuracy has been misleading if not completely inaccurate (sometimes intentionally so). Former President Donald Trump’s rhetoric downplaying the danger of the coronavirus inspired many right-wing news outlets, for instance, to question the extent of the pandemic and the response to it, including the accuracy of testing.

It can be confusing to have the coronavirus and feel OK, given that so many people become seriously ill with Covid-19. Wishful thinking plays a role, too. The belief that you don’t have to upend your life to isolate or to worry about infecting others is extremely attractive, and people tend to underestimate their personal risk.

If you think — despite all this — that your positive test result is mistaken, you can, of course, get a second test, but isolate while you’re waiting for the result. The much more likely case, however, is that you have the coronavirus and don’t have symptoms of Covid yet or are among those who do not become seriously ill. “If you’re asymptomatic and truly infected with Covid,” Dr. Gronvall said, “consider yourself lucky.”


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