With school back in session next month, many are worried about kids and coronavirus — not just how easily children become sick with COVID-19, but also how easily they spread the virus. A new study from Mass General Hospital seems to show the concern is warranted. It suggests that kids who contract coronavirus can carry high levels of the virus, even when they don't exhibit symptoms. The study's lead author, Dr. Alessio Fasano, spoke with WGBH's All Things Considered Anchor Arun Rath. The transcript below has been edited for clarity.

Arun Rath: There have been some questions overall about how susceptible kids are to the coronavirus. Tell us what your study found.

Dr. Alessio Fasano: We embarked on this study because we were trying to understand why kids were spared by the COVID-19 pandemic; that was our sense. It was a major surprise when we did the surveillance on almost 200 kids and found a sizable percentage of them that turns out to be positive, with the presence of a high load of viral particles in the upper airways. As high, if not higher, than the ones we detected in adults that were admitted to the hospital and the intensive care unit. And again, with little-to-no symptoms.

Rath: Is that unique for children? We've also heard recently that it seems like there might be more people who are carrying the virus who we also haven't tested.

Fasano: No, I think it's not unique for kids. It's just this missed part of the iceberg of this pandemic. The only people that we had been testing so far, given the limitation of swabs and testing tools, are the people that come down with the severe symptoms. Those are the ones that have been the objects of intensive scrutiny. But I'm pretty sure, as has been said over and over again, that the ones that are officially diagnosed are just the tip of the iceberg.

Rath: Talk about what the study shows in terms of how children can be spreaders of the virus as well.

Fasano: That's right. The study, for the first time, proved that not only are they having high viral loads in their upper airways, as was also proven by other studies, but we saw viral particles that can be vital and can be spread. So if they're there in high numbers, it is only obvious to suppose — we did not prove — but to suppose that if these kids cough or sneeze, these viral particles will get into the air and eventually be caught by somebody susceptible to the infection.

Rath: Something else we seem to have learned recently was the degree to which this virus can be maintained in the air, in aerosolized form.

Fasano: That's right, so we need to be aware of this information that was not clear before. We were not entirely clear if these kids were spared, for whatever reason, by this infection. One of the hypotheses was that they don't express the receptor for this virus, so the virus cannot infect the kids. The study proved otherwise, that this receptor is expressed and therefore it makes sense that you find viral particles in the upper airways of these kids.

Rath: Critics of your study, if I understand correctly, say it wasn't looking if there was live virus inside a person, just its genetic material. As such, there may be a discrepancy between that and how much a child would be infectious with a live virus. Can you respond to that?

Fasano: Yeah. When we do swabs, what we're looking at is the genetic material that is a marker of this SARS-CoV-2 virus. What we did was not a genetic test, we actually counted the number of viral particles in the airways, so they are supposed to be alive. This is the real lead of the fact that not only are kids infected, they're infected with viable particles.

Rath: Is there anything in the study that gives us a sense of why children, it seems, tend not to get that ill, showing no symptoms at all, even if they do carry the virus?

Fasano: The other history of COVID-19 — and keep in mind this is a brand new virus so we're learning on the go here, it's like we're building the airplane while we're flying — is probably that this infection has three stages. The first one is when the virus encounters a host and it translates to the colonization of the upper airways. That results in little to no symptoms, and this is common to kids and adults alike. Then there is a second stage that seems to be peculiar only to adults — the virus travels from the upper airways down to the lower airways and the lungs, and there they cause a severe pneumonia that brings these people to be admitted to the ICU, because they can't breathe anymore. That seems to spare kids, and we don't know why. There is a third, rare, but very concerning stage, that happens weeks or months after the infection, in which the immune response will build up this cytokine storm that can affect many organs, like the heart, the brain, and the kidneys. It has this multistate inflammation that is a scary complication of the infection, and kids experience this as well.

Rath: Obviously is in the context of us talking about kids going back to school, a very clear and present public health context for this. I know a lot of parents, myself included, even before the study have been having conversations about when our kids are back in class, how much we should limit our contact with our other relatives, because they'll be in that bigger pool of people. Could you put this in a deeper context for that, for those concerns? What should we be thinking about now as we are considering putting kids physically back into classrooms?

Fasano: We are scientists and clinicians, we provide data. It is up to the policymakers to make good use of these data, and not just based on hearsay. Now, you have the physical evidence that these kids can be infected and potentially can spread the virus. So if school needs to be reopened, this information needs to be taken into consideration. The three pillars that have been so far extremely effective in controlling the spread of the virus: physical distance, wearing a mask, and frequent hand hygiene are the three pillars that need to be implemented if we do in-person school reopenings.

Rath: I hate to put you on the spot since you are a researcher, but I'm wondering, should parents feel nervous about, if it is an option for them, sending their kids back into classrooms?

Fasano: Should parents be nervous to go to a supermarket or to eventually go into a restaurant that's now reopening? Sure, but only if you don't implement these guidelines. I think that there is a reality there we are facing. There is a virus that is new to us and therefore we are all susceptible. We have been in a lockdown to try to minimize the spread of the virus. Some states have been more effective than others in bending the curve. What is the next step? Of course we can be locked down forever, or until we get the vaccine that comes. Of the two, if we are confident, extremely confident, that school principals and legislators take this very seriously and implement a safe way for the kids to go to school, I think it's a calculated risk, but the risk is still there. If we are not confident that these rules will be implemented for whatever reason, and I heard that some school districts will implement the physical distancing but not using the masks, then definitely that will be a reason for concern, at least for me as a parent.

Rath: With this data that we have now, how should we incorporate that into our understanding the way that we do with other points of data in understanding this disease and how we proceed with it?

Fasano: There are so many questions that remain unanswered, and again, I think this is the foundation for the next wave of research — to try to understand why, for example kids don't get pneumonia, why kids that develop this complicated outcome of the infection will have this huge immune response. They have a very high tide of antibodies against the virus. What does that mean? The fact that the kids we found infected mostly, disproportionately, belonged to lower socioeconomic families. And they pay the higher price for this. What does this mean when they come back home where it's a multigenertaional household with grandparents there that can be exposed if they go to school and they bring this back? If those measures to limit the spread are not implemented, these are all the questions that we need to keep in mind moving forward.