By Sarah Gibbens

Since the beginning of the coronavirus pandemic, children have been largely spared the worst health impacts of COVID-19. The same SARS-CoV-2 virus capable of killing a 50-year-old might leave a four-year-old unscathed.

Now, the U.S. Centers for Disease Control and Prevention is recommending K-12 schools reopen this fall, saying the health risks should be weighed against the detriment of being kept home—which disproportionately impacts low-income and minority children and those with disabilities who may rely more on programs like school lunch and after-school care. When those children are kept from school, their grades grades slip, mental and physical health suffers, key times to socialize are lost, and many are falling behind on their routine vaccines.

“Children are suffering in different ways from adults,” says Megan Tschudy, a pediatrician at the Johns Hopkins University School of Medicine.

However, scientists are still struggling to understand how the virus affects children and whether kids can spread it to their older caregivers. Overall, scientists don’t fully understand why multiple kinds of coronaviruses—including COVID-19 and its viral cousins SARS and MERS—have different levels of severity across age ranges, says Rachel Graham, an epidemiologist at the University of North Carolina Chapel Hill.

Graham, who first spoke in March about COVID-19’s effects on children, says our understanding of why the virus seems to go easy on kids has not fundamentally advanced since then. Even with increased testing showing that more kids are capable of contracting the virus than we previously thought, experts can only theorize as to why children are largely spared the intense version of COVID-19 that strikes so many adults.

It’s also unclear how easily kids can spread the virus, both to each other and to adults. One robust study of nearly 65,000 kids published by the South Korean Center for Disease Control last week showed that children in the 10- to 19-year-old age range could spread COVID-19 within households just as effectively as adults.

According to the CDC, only 2 percent of domestic COVID-19 cases have occurred in children under 18, but data shows those rates can vary greatly by region. So far, 20 children under age five in the U.S. have died from COVID-19.

A small percentage of minors who test positive for COVID-19 develop a life-threatening condition called multisystem inflammatory syndrome in children (MIS-C), and it’s unclear if the disease has other long-term consequences.

“It leaves lasting scars on the lungs, and can lead to more severe illness down the line,” says Graham. But, she adds, “those kinds of things will have to be studied on a more long-term basis with children who have recovered from the disease.” And because youth have largely been spared such severe illness, “there’s been a lot less research about them and a lot less testing,” says Tschudy.

A large study is now underway in the United States to understand how COVID-19 infects children, even as parents and pediatricians are grappling with an upcoming school year scheduled to begin while infection rates continue rising.

How well can they spread the disease?

“All things being equal, kids are far more likely to transmit things,” says Graham, referencing the way children in particular tend to interact with each other and touch all kinds of objects and body parts. She notes, however, that there aren’t enough data to show that they do transmit the virus on par with adults.

While the study from South Korea showed that children over 10 effectively spread the virus, much younger kids were 72 percent less likely to spread the disease to adults.

However, it’s not out of the question that a child under 10 could transmit the virus. One study found very young children, including infants, left behind traces of the virus, though it’s not clear how infectious these remnants were. But another study tracked a COVID-19 positive nine-year-old who visited three schools without transmitting the virus. How children are managed seems to play a large role in transmission. Childcare centers that remained open during the pandemic have had a range of experiences, from large outbreaks at camps to infection-free daycare centers.

One theory for why children may be less likely to spread the disease to others has to do with the fact that COVID-19 primarily spreads through the droplets you breathe out, and children may breathe out with less force, and closer to the ground.

“If you have children, you know they can shriek pretty loudly, but that may not translate into a farther spread,” Barnett says, considering that a kid’s loud pitch lacks the full force of an adult cough or sneeze.

“Although they do congregate, they’re not packed into things like the New York subway or bars or sporting events in quite the same way,” she notes.

And where a sick adult might be likely to commute into a shared office, parents are often careful to keep their sick children home.

Ultimately, Barnett notes, experts can only offer theories.

“One thing that would make this a lot easier to understand are results from contact tracing,” notes Graham. “Those would give a much better idea of how many people are coming into contact with each other.”

Why don’t under-10s seem to get as sick?

“In the beginning of the pandemic, there was so little that was known for all ages,” says Tschudy. “It was assumed that all ages might be equally affected, and there was a huge amount of preparation.” Early school closures, she says, may have played a part in shielding kids from the virus.

Testing was also limited to people showing visible signs of a possible COVID-19 infection, and Tschudy says kids who were infected and asymptomatic likely slipped by unnoticed.

One prevailing theory for children under 10 don’t seem to get as sick has to do with an enzyme called ACE2. When SARS-CoV-2 enters the body, the spikey proteins encircling the virus latch on to ACE2 like a key fitting into a lock.

“One of the theories is that children have the [ACE2] receptors for this virus more in the nose [and] in the upper respiratory system than in the lungs, and adults have these receptors in the lungs,” says Elizabeth Barnett, chief of pediatric infectious diseases at Boston Medical Center and professor of pediatrics at Boston University School of Medicine.

Producing more ACE2 receptors in the lungs is one theory for why adults have more serious COVID-19 infections, she notes.

One study of 305 people from four to 60 years old found that ACE2 enzymes were least active in children under 10.

More resilient and adaptive immune systems may also help young children fend off the disease, says Alvaro Moreira, a neonatologist at the University of Texas Health Science Center in San Antonio. He describes two methods of attack used by a person’s immune system: “one that doesn’t require memory and one that does.”

Over time, as we age and get exposed to bacteria and viruses, our bodies' immune systems create cells that remember specific viruses and can later attack them more efficiently. A child’s body that’s still building this memory relies on the immune system’s other method of attack.

“That’s the innate immune system,” says Moreira. “And we know children are less likely to mount an exaggerated innate response.”

When the innate immune system attacks, immune cells indiscriminately take on pathogens that enter the body. It’s also during this onslaught that the body releases molecules called cytokines, which help cells communicate with one another. When the immune system unleashes too many cytokines, they attack healthy tissue. Some of the sickest adult COVID-19 patients have died from these so called “cytokine storms.”

Kids tend to have lower cytokine levels to protect them from such storms, says Tschudy, possibly because, “young children are exposed to new infections all the time, so when their bodies are exposed to a new virus like COVID-19, their immune systems may be primed to respond just strong enough to fight the virus and not cause their bodies harm."

Some children face higher risks

While a child’s immune system might seem biologically primed to ince the beginning of the coronavirus pandemic, children have been largely spared the worst health impacts of COVID-19. The same SARS-CoV-2 virus capable of killing a 50-year-old might leave a four-year-old unscathed.

“The vast majority of children with severe COVID tend to have other risk factors,” says Philip Zachariah, a pediatric infectious disease specialist at Columbia University and epidemiologist at New York-Presbyterian Morgan Stanley Children's Hospital.

In a study he published in early June, Zachariah reviewed the cases of 50 children who were admitted for COVID-19. All but one child recovered. Obesity in children over two was associated with more severe manifestations of the disease, though Zachariah emphasizes that this may simply reflect the neighborhoods served by New York-Presbyterian.

“I think the data is generally consistent with the fact that lower-income kids and racial minorities are infected more,” he says.

Overall, he says, even young kids who do get sick seem more likely to recover than sick adults. And the same ways adults stay safe—social distancing, wearing masks, and hand washing—will ultimately help kids contain the virus.