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Tuesday, April 16, 2024

You Should Still Test Yourself for Covid. Here’s When

It’s been a few weeks since the federal government began mailing out the free Covid-19 testing kits promised by the Biden administration, and it’s finally become easier to find rapid tests in stores. But even with Omicron’s surge on the decline, the pandemic isn’t over. Concerns remain about Omicron’s sister, and a new variant can always arrive. So what’s the most efficient way to use the government’s free tests? Whether you’re vaccinated, boosted, unvaccinated, or recovered, when should you test for Covid-19 and how should you interpret the findings?

Like so much of the advice and data during the pandemic, it’s complicated. When you should test and what the results mean—or don’t mean—depend on your individual circumstances. ”You can’t just have one rule that applies to everyone because there is variation around test results,” says Esther Babady, chief of the clinical microbiology service at Memorial Sloan Kettering Cancer Center in New York. “We like to make recommendations with data, and the thing that has been so frustrating with the pandemic is that you just don't have enough time to gather the amount of data that we are used to getting before we can say anything.”

And what we say today might be different several weeks from now, points out Susan Butler-Wu, director of clinical microbiology and an associate professor of clinical pathology at the University of Southern California Keck School of Medicine in Los Angeles.

“The calculus changes at various points in the pandemic,” Butler-Wu says. When Omicron rates were raging, going to a bar or restaurant could practically guarantee that you’d be there with someone who had Covid-19 and didn’t know it. But, she adds, most people aren’t staying home full-time anymore and aren’t likely to wear a mask around their family because they went to a bar. We have to consider what’s practical too, she says.

We’ll start with the handful of clear-cut situations when you should definitely test. Most of this advice focuses on rapid antigen tests, but we’ll also note when to consider PCR testing and why.

Test if You Have Symptoms

If you have Covid-19 symptoms, test yourself no matter what your vaccination status or Covid-19 history is. You don’t need to test for every headache you have, but fever, cough, or a stuffy or runny nose should definitely trigger a test. It might be annoying during the winter when flu, colds, and other viruses are also circulating, not to mention the allergies that will descend on many people as spring draws closer. But if you blow off that sore throat or sniffle or cough and don’t test, especially if you’re not wearing a mask around others, you risk infecting someone’s grandmother or child or father or … you get the idea. Even if vaccination rates are high in your area, plenty of vaccinated, boosted people are immune-compromised or otherwise high risk. Exposing them could kill them. So, if you have symptoms, be a good human and test.

Test if You’ll Be Traveling

If you’re flying internationally, check the destination country’s requirements—you’ll probably need to show a negative test result, even if you’re vaccinated. If you fly into the US, you’re required to show a negative test result or proof of a recent Covid-19 recovery, regardless of your vaccination or citizen status. Even if you’re traveling within the US, the CDC recommends that anyone not fully up to date with Covid-19 vaccination (including boosters) get tested before leaving.

Test if You’ll Be Around Someone Particularly Vulnerable

Many people have returned to normal or mostly normal lives—visiting restaurants and bars and coffee shops, going to the movies, gathering with friends, etc.—but millions of immune-compromised individuals still have to be extra careful. Some don’t build up enough antibodies in response to the vaccine. Others might have antibodies but an immune system too weak to effectively fight off a breakthrough infection.

If you’ve been living a normal-ish life, especially without wearing a mask or taking other precautions, you should test before visiting anyone at particularly high risk, even if you don’t have symptoms. High-risk folks include organ transplant recipients, people undergoing active cancer treatment or living with a blood cancer, people with immune-compromising conditions or taking immune-suppressing medication, people particularly frail and elderly, and anyone with multiple serious underlying conditions that, even if vaccinated, makes them high risk with a breakthrough infection. Test twice 24 hours apart if you use a rapid test, but if you have any symptoms or think you might have been exposed and you can’t delay your visit, consider a PCR test, Babady says.

“If the person I'm going to see does not have a regular immune system, or even if they were vaccinated, I'm going to want to use the most sensitive test possible to make sure I'm not exposing that person,” Babady says.

Test When You Feel Recovered From Covid-19

If you had Covid-19 but your symptoms have subsided, test to confirm you really are Covid-19-free. PCR tests can remain positive days or sometimes weeks after you’re no longer infectious, but a negative rapid antigen test means it’s less likely you’re still infectious. Your school or work might also require a negative test to return.

Also keep in mind that, regardless of CDC recommendations, there’s a good chance you’re still infectious more than five days after initially testing positive, another reason to test before rejoining the world. One study, for example, found that half of people infected with Omicron still had high viral loads at day 5, suggesting they were likely still contagious.

What if I’ve Been Exposed?

If you know you’ve been exposed, test five days after the exposure. What you do during those five days depends on your vaccination and infection history.

If you’re unvaccinated, or vaccinated but not boosted, and you haven’t had Covid-19 in the past three months, you should quarantine at home during those five days and wear a high-quality mask around household members—an N95, KN95, KF94, or similar respirator mask that fits well. If you must leave the house, wear the best mask you can find and make sure it fits. And stay away from others.

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If you’re vaccinated and boosted, you don’t need to stay home, but you should still test five days after the exposure, symptoms or not. (And definitely wear a mask in public.) You may want to test again a few days later, Babady says, because it might take longer for a positive result in a vaccinated person if their viral load is lower (we don’t have the data to know either way). For example, one small preprint study found that most Omicron cases were infectious a few days before rapid antigen tests showed positive. If your results are negative but you develop symptoms within 10 days of your last exposure, stay home, wear a mask, and test again.

What’s an “Exposure”?

The CDC definition of exposure is about as helpful as a space heater in hell: “Contact with someone infected with SARS-CoV-2, the virus that causes Covid-19, in a way that increases the likelihood of getting infected with the virus.” What that really means is having close contact, defined by the CDC as being within 6 feet of an infected person for at least 15 minutes total starting two days before the person developed symptoms—even if both of you were wearing masks.

Unless, of course, you’re a student in school. Then, the definition of a close contact is … more complicated. A close contact for students means being within 3 feet of the infected person for at least 15 minutes, regardless of masking, or within 6 feet if either person was maskless. The difference in definitions is necessary when kids have to sit within 6 feet of each other to fit in a classroom and not have to quarantine—and lose learning time—every time there’s an infection.

But the virus doesn’t care whether it’s in a classroom or its potential host is a student or adult. The CDC definition might make sense for school attendance, but it’s less helpful for parents unsure whether to test their kid after the kid sat five feet away from a classmate with Covid-19 for seven hours. Many parents will want to test in that situation—t’s a judgment call.

And the problems with determining “exposure” don’t stop there. The more you go out in public, especially in enclosed spaces, the more your chances of encountering someone infected increase, especially if neither of you is wearing a mask.

So should you just routinely test yourself every so often? No, that’s a waste of testing, not to mention expensive and probably nerve-racking. Test when the situation calls for it, and spend the rest of your pandemic-related energy reducing the likelihood that you’ll need to test, Babady says.

What if My Test Is Negative?

On rapid tests, false negatives are more likely than false positives. If your area has a lot of Covid-19 and you have symptoms, there’s a decent chance it’s a false negative. “A lot” means substantial or high transmission, a 7-day average of more than 50 new cases per 100,000 people or a test positivity over 8 percent.

In that case, assume you’re positive until you test again 24 to 36 hours later, depending on the test kit recommendations. Babady says she might even test three times if you clearly have Covid-19 symptoms in an area with high transmission. False negatives in symptomatic people were especially likely during Omicron’s peak, not only with the high case counts but also because of the particular disease dynamics of Omicron, which seems to take longer to deliver a positive result. Once transmission dies down (or another variant comes along) that calculation may change.

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But false negatives are rarer with PCR testing, Babady says. “You’re amplifying the genetic material, so even when there’s trace amounts of it, you increase the total detectable amounts so you can pick it up,” she says. If you have a negative PCR after a negative rapid test, you can feel confident you don’t have Covid-19.

Even with a negative test, though, unless you know it’s allergies, you probably have some other respiratory virus. If you’ll be around others, wear a mask until you’re no longer sick so you don’t give someone else the flu or another illness. Even if you’re healthy, plenty of others may not be able to fight off an infection as well as you can.

Can Rapid Tests Tell Me Whether It’s Safe to Go Out?

Despite the popular use of rapid antigen tests as “screening” measures to allow folks to gather without masks, this approach makes assumptions that we don’t have the data to back up—and that’s never really been an appropriate use of tests, Butler-Wu says.

“They're continually referred to in the public discourse as tests for infectiousness or tests for contagiousness,” but experts in clinical microbiology balk at that idea “because there's no such thing as a test for infectiousness or contagiousness,” Butler-Wu says. “These test for high viral load,” one component of infectiousness but not the only one, she says. For example, if you test negative because you have an infection but your viral load is just shy of what the test picks up, that doesn’t mean you aren’t contagious, especially if you’ll be maskless in a small, poorly ventilated room.

Plus, rapid tests were designed to assess the presence of an infection in people with symptoms, not people without symptoms. If you use rapid tests as a “screening” tool among asymptomatic people, negative results may mean a lower likelihood that someone present has a contagious infection, but it’s far from a guarantee.

“I’ve never felt comfortable with people deciding it was somehow safe to do something on the basis of a negative test and not going along with the usual precautions,” Butler-Wu says. “You can get away with that when the community prevalence low because, statistically, it's unlikely that the people meeting have it. That becomes a completely different kettle of fish when the prevalence starts to increase, and now you're seeing it with Omicron.”

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