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New Study Says 'Silent Spreaders' May Be Responsible For Half Of U.S. COVID-19 Cases


Silent spreaders - people who have the coronavirus but haven't yet developed COVID-19 symptoms or may never develop them. A new study suggests they may be responsible for half of the infections in the United States. Alison Galvani led the study. She's a professor of epidemiology at Yale University. And she joins us now. Welcome.

ALISON GALVANI: Thank you for inviting me, Lulu.

GARCIA-NAVARRO: What was our previous understanding? I mean, what did epidemiologists like yourself think before now?

GALVANI: Well, many diseases such as Ebola - people tend not to become infectious until they're symptomatic. So with Ebola, people don't become really infectious until they're extremely sick. An unusual aspect of COVID-19 is that the peak of infectiousness occurs during the pre-symptomatic phase. So the average incubation period prior to symptoms is about five days, the last two days of which constitutes a highly infectious pre-symptomatic stage.

GARCIA-NAVARRO: So there are these two classes of silent spreaders, right? - the pre-symptomatic people and then the asymptomatic people. What does your research say about these two populations? Which one is bigger? Which one is smaller?

GALVANI: Well, the pre-symptomatic stage is more important because everyone passes through the pre-symptomatic stage. And it's just a question of whether you remain symptomless or go on to develop symptoms. One of the - well, there's two studies that had large sample sizes. And in one case, they found about 17% was asymptomatic. And another case - about 30% was asymptomatic. We used those scenarios in our modeling study.

GARCIA-NAVARRO: What do your findings imply for how the U.S. needs to respond to this outbreak? Because if we have such a high spread from people that we don't know actually have the illness, that is incredibly worrying for how we might contain it.

GALVANI: Absolutely. You know, unfortunately, temperature checks will not be a panacea because by the time people develop a temperature - if they develop a temperature - they will have already passed through the highest period of their infectiousness. So our analyses underscore the importance of contact tracing and testing that is fast enough and extensive enough to identify pre-symptomatic cases prior to the onset of symptoms.

GARCIA-NAVARRO: So what's your advice to people right now? Or better yet, how are you conducting yourself in a society that your research says is full of silent spreaders? I mean, do you look at everyone and think they're potentially infected?

GALVANI: I think that's the reality - that they potentially could be. Even if somebody seems young and healthy - in fact, you know, given that young people are disproportionally responsible for silent COVID-19 transmission, reopening schools would be adding fuel to the fire. Even if all symptomatic cases were kept at home, our study shows that a widespread outbreak may nonetheless unfold from silent transmission.

You know, I have three young children myself. And I appreciate the priority that education is. However, from a public health perspective, I think it would be reckless to reopen schools until we have dramatically suppressed the number of daily cases and have the capacity to conduct sufficient contact tracing.

GARCIA-NAVARRO: So you're not going to send your kids to school in the fall?

GALVANI: No, I don't think so, I mean, unless things really change over the summer. But we're not going in the right direction. And we don't have the federal leadership that we need right now.

GARCIA-NAVARRO: That's Alison Galvani, a professor of epidemiology at Yale University. Thank you very much.

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

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