Tanya Lewis: Hi, this is Your Health, Quickly, a Scientific American podcast series!
Josh Fischman: We bring you the latest vital health news: Discoveries that affect your body and your mind.
Lewis: And we break down the medical research to help you stay healthy. I’m Tanya Lewis.
Fischman: I’m Josh Fischman.
Lewis: We’re Scientific American’s senior health editors.
In today’s episode, we want to talk about some of the current challenges with using home COVID tests. When you first have symptoms, a change in how your body reacts to the virus could lead to a test result showing you’re negative when you’re actually infected.
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Fischman: You know how we try and give people easier ways to understand and use medical research?
Lewis: Yeah. Simplifying complex science is kind of our job.
Fischman: Well, today we’re taking something that seemed simple and we’re going to say that it’s actually more complicated now. We’re going to say that with COVID home tests–the rapid antigen ones–it’s become a little tougher to get an accurate result.
Lewis: That’s bad news, especially if, like me, you use those tests a lot.
Fischman: You should still use them. They’re very good at telling you if you have COVID. But telling you that you don’t have COVID, right when you start to feel symptoms—that isn’t as easy as it used to be.
Lewis: You mean that it’s possible to get what scientists call a false negative. The tests might say you don’t have COVID when you really do.
Fischman: Yeah, that’s what’s going on here. You can still use the tests accurately. But it might take a few days to get a good answer.
Lewis: Back in the middle of the pandemic, you would start to feel COVID symptoms— a sore throat, a runny rose, a fever, trouble breathing. And you’d reach for an antigen test that same day, swab your nose, put a few drops in the test kit, and wait 15 minutes. And if you didn’t see a pink line show up, you’d breathe a sigh of relief.
I mean you’d still be sick–you might have the flu, or a cold virus, or RSV–but you wouldn’t be infected with the SARS-CoV-2 virus that causes COVID. And that was a relief because COVID has killed more than a million people in the U.S.
Fischman: And it’s still killing people, because we’re in another winter holiday wave. In December it killed about 1500 people per week, according to CDCs data.
Lewis: But why is it easier to get a false negative now? Have the tests changed?
Fischman: No. But we’ve changed. Our bodies are reacting faster to the virus now. And that means we’re getting symptoms faster, before there’s a lot of virus inside of us. Our colleague, SciAm health editor Lauren Young, has been reporting a story on this.
Lauren Young: It’s a zoo of trying to figure out what you have. Like, what is the menagerie of viruses right now? And there’s plenty of them. And there have been reports lately of people testing and saying that they have symptoms. They take, you know, these handy antigen tests that we’ve been getting, and they’re testing and they come back negative.
Fischman: But they’re not really negative, Lauren says. People will test a few days later and will come back positive. So they really have had COVID. And there’s a theory about why their first tests were negative.
Young: The possible idea for this is that our immune systems are kind of primed to identify this virus, whether you’ve had a previous infection, or you’ve been vaccinated. According to Peter Chin-Hong at UCSF, who told me this, your antibodies are kind of like sharks just waiting for, you know, any sign of COVID virus in your body, and the moment it detects that, it’ll flare up an immune response. So even if your viral loads, or your viral levels in your body are quite low, your body’s still reacting.
Lewis: You know I think I had something like this recently where I had all the symptoms of COVID, I got a fever, I felt achy, and all these other things, but then I never tested positive, so I think I must have just had a very low viral load, and my body was reacting. And it turns out that when your body is fighting an infection, your immune system causes inflammation. This leads to symptoms like congestion and a sore throat. So your symptoms are basically created by your own immune response, not by the virus itself.
Fischman: And a lot of people have been vaccinated, or they’ve had COVID infections, so their immune system is responding much more quickly than it did early in the pandemic.
Young: We’ve been told, like, once you have symptoms, you should test. But your viral loads might not be high enough for that test to come out positive. So you might be thinking, you’re in the clear, because you’re like, “Hey, okay, I have symptoms, my test is negative, I must have some other, you know, illness, respiratory illness,” you should still stay home, if you have any kind of respiratory illness. But a lot of researchers are saying that maybe the timeline should be a little bit shifted in terms of when you take that test. Or if you do have a negative test, and you’re still experiencing symptoms, maybe wait another 48 hours and test again.
Lewis: In fact, there was a paper in the journal Clinical Infectious Diseases, published in September, that compared test results to time of symptom onset. On the first day of symptoms, there was only a 30 to 60 percent chance that the test would be positive. The likelihood increased each day. So by day four it was up to 93 percent.
Fischman: Okay, but that makes things harder on people. If you’re sneezing but you need to wait 3 days until you know whether you have COVID, what do you do in the meantime? Do you isolate from your family or not? Do you stay home from school?
Lewis: Yeah that’s a great question. The CDC guidelines are 5 days of isolation after symptoms start, and then you can go out—but wear a mask. After 10 days total, you can ditch the mask. And positive results are still accurate early on. So if you get symptoms, take a test, and if it’s positive, yes, you should definitely isolate and assume you have COVID.
Fischman: Yeah, that’s important to remember about the accuracy of the positive results. Also, the negative results are pretty accurate after day 5. There was a study in JAMA Network Open that looked at people with negative tests on day 6. Of those people, none had virus particles that could replicate and get into other cells. In other words, these weren’t false negatives. These folks couldn’t infect other people.
Lewis: But if you’re at higher risk of severe disease–if you’re older or immunocompromised, for instance–and need an answer sooner than five days, you should see your doctor. You might be able to get a PCR test, which can detect infections sooner than antigen tests. And if you’re positive, you might qualify for Paxlovid or another antiviral drug. But you have to take it within the first five days for it to be effective.
Fischman: That’s good to know. But if you get a negative test when symptoms start, now you’re in this limbo period. You should probably isolate until you can take another test in 2 or 3 days. And those tests add up. Governments aren’t handing out as many free ones as they used to, and tests cost $20 or $25 at the drug store.
Lewis: Right. But there are still ways to get your hands on free tests. Freelance science writer Sam Jones recently reported in Sciam about some of these methods. The U.S. postal service still mails out free tests to households that request them. And the Health Resources and Services Administration makes free or low-cost COVID tests available to people who are uninsured or members of underserved communities through public health centers.
Fischman: Also the National Institutes of Health has a Home Test to Treat telehealth program for underserved groups. It provides free COVID home tests for people who haven’t already tested positive. And for people who are already ill, the program offers treatment options like the drug Paxlovid. We’ll put links to all these services in the transcript for this episode.
Lewis: So COVID testing may be a little more complex now. But with a little extra care, you can still keep yourself and the people around you safe.
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Fischman: Your Health, Quickly is produced by Tulika Bose, Jeff DelViscio, Kelso Harper, Carin Leong, and by us. It’s edited by Elah Feder and Alexa Lim. Our music is composed by Dominic Smith.
Lewis: Our show is a part of Scientific American’s podcast, Science, Quickly. Subscribe wherever you get your podcasts. If you like the show, give us a rating or review!
And if you have a topic you want us to cover, you can email us at Yourhealthquickly@sciam.com. That’s your health quickly at S-C-I-A-M dot com.
For Your Health Quickly, I’m Tanya Lewis.
Fischman: And I’m Josh Fischman.
Lewis: See you next time.
