You can still be contagious with COVID if you have a negative test — here's why

Shiv Sudhakar, MD
·7 min read
2.2k

As the nation experiences what many experts believe is the second-largest wave of COVID infections since the pandemic started, many Americans will be checking to make sure they don’t have the respiratory illness.

COVID testing guidelines and what we know about how long you're contagious have changed since the start of the pandemic. So we sat down with a leading epidemiologist, who provided guidance on which tests to do, when to do them and how to interpret them.

When should you test for COVID?

If you have COVID symptoms, you should take a test immediately, according to the U.S. Centers for Disease Control and Prevention.

If you were exposed to COVID, you should take a test at least five days after your exposure.

If you don't have symptoms or any known COVID exposures, you can may also consider testing before an even where you'll encounter a lot of people or if you're spending time with someone high risk for severe illness, such as an older or immunocompromised person. Test right before the event or visit, if possible.

How accurate are COVID tests now?

A positive result on an at-home COVID test is very reliable, according to the CDC. However, a single negative result with an at-home test may not be accurate because you may have taken it before the virus reached detectable levels.

That's why, if you're using at-home tests to detect an infection, you should test more than once.

If you have symptoms and test negative with an at-home rapid test, test again 48 hours later, the CDC advises. If you were exposed to COVID, do not have symptoms and test negative, test again 48 hours later. If that test is negative, test again another 48 hours later.

The emergence of new variants, in particular JN.1, has not affected the accuracy of at-home tests, TODAY.com previously reported.

If you want to take only one test, the CDC recommends what's known as PCR test for the most reliable result. PCR tests are usually administered in medical settings, and they detect a virus's RNA, which is similar to human DNA, Dr. Michael Mina, a leading epidemiologist and chief science officer at the telehealth company eMed in Miami, Florida, tells TODAY.com. (At-home tests are usually antigen tests, which look for proteins of the virus.)

He notes that PCR tests often stay positive for days or even weeks longer than people are contagious, making them ideal for diagnosing COVID, but less ideal for knowing when you no longer need to worry about spreading an infection to others.

Can you be contagious after a negative COVID test?

If you test negative with a PCR test, you are likely not contagious.

But if you test negative with an at-home test, the answer will depend in part "on whether the negative COVID test is at the beginning of feeling sick or on the way to recovery,” Mina says.

“If you have already been positive and are testing to see if you are recovering or recovered, then as soon as you become negative, it is appropriate to assume you are no longer infectious," he explains.

When a positive rapid antigen test goes from a dark line to a very faint line, this means that the virus load in the swab is probably less than when then line was dark, he adds.

“So even a faint line after a really dark line means you are likely much less contagious, and no line means you are likely very low risk of being infectious," Mina says.

But at the beginning of an COVID illness, an at-home antigen may come back negative, even though you may become infectious as the viral load increases.

“You may be starting to feel symptoms because your immune system is activating, but the virus might not yet be high enough in your nose to cause a test to turn positive," Mina says. In this scenario, you may test positive several hours later, the next day or the day after that.

If you get a negative at-home test result at the beginning of a possible infection, keep the following guidance from the U.S. Food and Drug Administration in mind when weighing your risk of having COVID:

  • If you have typical symptoms with a known exposure, assume you have COVID (despite the negative result). Take precautions and test again 48 hours later.

  • If you have typical symptoms but no known exposure, you might have either COVID or another illness. Take precautions and test again 48 hours later.

  • If you have no COVID symptoms but a known exposure, you might still have COVID. Take precautions, test again 48 hours later, and if the second test is negative, take a third test 48 hours later.

  • If you have no COVID symptoms without any known exposure, you probably don’t have COVID. Test again 48 hours later, and if it is negative, take another 48 hours after that.

When are you no longer contagious from COVID?

If you get a negative at-home COVID test result after previously testing positive, you are likely no longer contagious, Mina says.

But how long that will take is "wholly dependent on the person,” he explains says. How long you are contagious depends on:

  • Your underlying medical problems

  • Your immunization status

  • Severity of your illness

  • The predominant circulating variant at the time

If you have mild illness or no symptoms, you're less likely to be contagious after day five of your illness (with day 0 being the day your symptoms started or you tested positive if you have no symptoms), per the CDC.

If you have moderate to severe illness, you may have to wait 10 to 20 days after your symptoms started to no longer be contagious. It may take people who are immunocompromised 20 days or more to no longer be infectious.

If you continue to test positive for COVID after 10 days, continue to take precautions until you have a negative test result, experts previously told TODAY.com.

How long should you isolate and mask if you have COVID?

The CDC recommends using its isolation calculator to determine how long you should take precautions. Here’s a summary:

  • No symptoms: Stay at home for at least five days but wear a mask when around others in at home.

  • Symptoms improving: End isolation after five days (as long as fever-free for 24 hours without fever-reducing medication).

  • Moderate illness (like breathing difficulty): Isolate for 10 days.

  • Symptoms not improving: Isolate until your symptoms are improving and you have had no fever for 24 hours (without the use of any medication to reduce fevers).

  • Severe illness (hospitalized or have a weakened immune system): Isolate for 10 days but check with your doctor first before ending isolation.

Regardless of when you stop isolating, the CDC advises wearing a mask around other people through day 10 of your illness — unless you get two negative antigen test results 48 hours apart prior to day 10. (In Mina's opinion, one negative antigen test after previously testing positive is sufficient to know you're no longer infectious.)

Mina also provides these examples of using rapid antigen testing to see when you can end isolation.

  • If you test yourself and you’re positive, stay in isolation (even if it's after day 10).

  • If you have access to several tests, consider repeating the test several days after you turn positive on the test. If your repeat test is negative, you can likely exit isolation — assuming you don’t have any symptoms anymore and fever-free.

  • If you have multiple tests with a faint positive line over several days, "you’re probably have very, very, very low infectivity, if at all,” Mina says, adding, "But maybe wear a mask, maybe don’t go to a cancer hospital or a nursing home, but you’re probably good to go back out and be with your family."

This article was originally published on TODAY.com