from Here's what COVID-19 does to a child's body (
National Geographic, 7/24/2020):
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."