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Infectious Disease Specialist Discusses Whether The U.S. Is Over-Testing For COVID-19


Early in the pandemic, there were not enough coronavirus tests to keep track of the disease. Now the U.S. might be facing something like the opposite problem. Dr. Monica Gandhi argues that too many vaccinated people are getting tested and finding out they have a breakthrough infection even if they don't have any symptoms. Dr. Gandhi studies infectious diseases at the University of California San Francisco and joins us to explain.



SHAPIRO: To be clear, you're not talking here about vaccinated people who fall ill, which we've all heard about. You're talking about vaccinated people whose bodies fight off the infection, but they still test positive for the coronavirus. How does that work?

GANDHI: Right. So how that works is it really has to do with the nature of the gold standard tests that we use in the United States, which is PCR. PCR is a very, very sensitive test. You can have literally one viral particle in your nose or a dead virus, and it will still amplify it. And so there are three levels of breakthrough, but this asymptomatic breakthrough - the problem with our PCR test is you can have a super-low viral load. You just killed that virus. You did exactly what you were supposed to do with your immune system. But the PCR test will still trigger positive. If you test someone who's asymptomatic, they may have actually had a major success with their vaccine. And that's a problem unless we actually measure the viral load that is in that PCR test.

SHAPIRO: Before we get to measuring that viral load, I want to disclose I'm one of the people you're talking about. I mean, over the weekend, I tested positive for the coronavirus because I was in contact with somebody else who tested positive. I have almost no symptoms. I'm now at home isolating. Isn't it useful for me to have that information so I can protect my husband, my colleagues, my friends?

GANDHI: So it is useful. But what would be even more useful is to know if you can pass it on to your husband, colleague and friends. And so what a viral load means, if you are mainly without symptoms, is - what we want to know about what was just swabbed in your nose is, what is your viral load?

When we do a PCR test, there's something called the cycle threshold of the test. It means that you have to go through lots and lots of cycles if you have a low viral load to trigger a positive. If you have a high cycle threshold, that means you have a low viral load. What you've just done is you've fought the virus in your nose, and you've protected everyone around you because you can't spread it. And so that's the important thing to know about an asymptomatic breakthrough - is we have to start going back to this concept of describing what your virus is so we know what to do with you. You may not have to isolate at all.

SHAPIRO: Does the health care system have the infrastructure to do the kind of analysis you're talking about? I mean, measuring viral load seems a lot more complicated than a straightforward yes, no, you have it, or you don't.

GANDHI: The fascinating thing is we absolutely have that infrastructure because a PCR test - literally, this is the nature of it. It has to go through a certain number of cycles, and then it goes, ding, ding, ding. And it triggers that you have a positive test or not. So that cycle threshold is actually part of every PCR test. We've just never incorporated that into our knowledge. And right now in the studying of breakthroughs, it's an important time to do it.

SHAPIRO: So you argue in a piece you wrote for The Washington Post that vaccinated people likely don't carry enough viral load to spread the infection to others. Do we really know enough about the delta variant to feel confident that that's the case?

GANDHI: So we don't. And in fact, I think the delta variant is a different beast. I think that it is something that has a higher viral load, which is why we are seeing now more symptomatic breakthroughs in people who are vaccinated - not that they're severe breakthroughs. It's still true that 99.6% of people in the hospital right now with SARS-CoV-2 in our country are unvaccinated. But there are more symptomatic breakthroughs. You're all hearing this - more of a mild cold, or even can feel like you have a flu. We need to know if those people can transmit. It's likely that they can, though, because there is higher viral loads with the delta variant - so another reason why viral load is incredibly important now when we discuss breakthroughs at this point with the delta variant.

SHAPIRO: Dr. Monica Gandhi studies infectious diseases at the University of California San Francisco.

Thank you so much.

GANDHI: Thank you. Transcript provided by NPR, Copyright NPR.

Ari Shapiro has been one of the hosts of All Things Considered, NPR's award-winning afternoon newsmagazine, since 2015. During his first two years on the program, listenership to All Things Considered grew at an unprecedented rate, with more people tuning in during a typical quarter-hour than any other program on the radio.
Courtney Dorning has been a Senior Editor for NPR's All Things Considered since November 2018. In that role, she's the lead editor for the daily show. Dorning is responsible for newsmaker interviews, lead news segments and the small, quirky features that are a hallmark of the network's flagship afternoon magazine program.