My 6-year-old has been exposed to SARS-CoV-2 at least four times and never tested positive. Many people fall into that category. Researchers have theories about why they've been able to ward it off.

Transcript

AILSA CHANG, HOST:

Two years into this pandemic, the majority of Americans have had COVID at least once, including myself, yet some people have continued to avoid getting infected even after being exposed. Scientists are beginning to understand why. And here to explain is NPR's Michaeleen Doucleff. Hi, Michaeleen.

MICHAELEEN DOUCLEFF, BYLINE: Hi, Ailsa.

CHANG: OK. So we have all heard this story of a family who gets really sick with COVID, but there's, like, one person in the family who doesn't even test positive for the virus, right? Like, that's what you're talking about here - not, like, some asymptomatic person.

DOUCLEFF: Right. Asymptomatic people still test positive, but there are people who've been exposed and never test positive. The virus isn't detectable in them. My young daughter actually seems to be one of these people.

CHANG: Lucky her.

DOUCLEFF: She's been exposed, like, four times, you know? And we're talking, like, heavy exposures, and she's never gotten infected. And I've been wondering why. Like, what is she doing?

CHANG: Right. And you're going to tell me that she's not just getting lucky, right? Like, there's something actually happening in her body that is protecting her.

DOUCLEFF: Yeah. So what scientists are starting to realize is that the immune system has a way, in some people, to clear out an infection in the earliest stages so quickly, in fact, the virus never reaches detectable levels. So although they are briefly infected, they never test positive.

CHANG: What's going on here? How does that happen?

DOUCLEFF: Yeah. Well, it looks like the body has a few tricks to stop infections, like, kind of in their tracks, and the first one is super-cool. It comes from a study published in the journal Nature. And in it, researchers essentially find that for some people, your exposure to other coronaviruses other than SARS-CoV-2 could actually help you fight a SARS-CoV-2 infection quickly.

CHANG: Wow. So these are, like, coronaviruses that cause the common cold.

DOUCLEFF: Right. Exactly. There are four of them that basically infect everyone. And I was talking to Brianne Barker about this. She's an immunologist at Drew University in New Jersey. She says they're not dangerous viruses. And in certain people, their immune system thinks these viruses look like SARS-CoV-2.

BRIANNE BARKER: You might imagine that if you had one of those common cold coronaviruses more recently, your immune response might be at a higher level when SARS-CoV-2 is encountered.

DOUCLEFF: Specifically, she says, your body generates immune cells to clear up these colds, and some of those cells stick around in your blood for a while. And there's evidence now that in about 10% of people, these immune cells may prepare the body to fight SARS-CoV-2.

CHANG: Dang. I wish I were one of those people. That's pretty cool. OK. So you said there were - there's, like, a few tricks. What else is there?

DOUCLEFF: Yeah. So this mechanism is super-important. Inside our respiratory tracks, there's this early warning system for viruses - many viruses. It's called RIG-I. And when it sees a new virus, it sets off this massive cascade of events which kills the infected cell and protects others from getting infected. For a lot of people, you know, that's not enough, and the virus overwhelms the system. But what scientists are realizing is that some people have very sensitive and powerful RIG-I systems. Donna Farber is an immunologist at Columbia University. She says this is probably one reason why kids, like my daughter, are so good at stomping out SARS-CoV-2.

DONNA FARBER: They're just adapted to respond to new pathogens, and they're ready to do it. They're ready to respond, and they do it more efficiently than we do.

DOUCLEFF: Because if you think about it, for adults, a new virus, like SARS-CoV-2, is really rare. We've seen a lot of viruses in our lifetime, and our immune systems don't have to be as on guard. But for kids, she says, basically every virus is new, and so their immune systems are pumped up to protect them from any new virus. And hopefully, that's what's been happening with my daughter.

CHANG: I certainly hope so. That is NPR's Michaeleen Doucleff. Thank you, Michaeleen.

DOUCLEFF: Thank you so much. Transcript provided by NPR, Copyright NPR.

Tags: T cells  covid vaccines  SARS-CoV-2  COVID-19  pandemic  antibodies  coronavirus  immune system