When innate immunity fails to control a virus, the body can fall back on the adaptive immune system, a second line of defense that adapts to each unique threat. The adaptive system creates antibodies, for example, tailored to each virus or bacterium the body encounters.
While antibodies are some of the easiest pieces of the immune response to measure, and therefore often cited as proxies for protection, kids don’t seem to need as many to fight COVID-19, Betsy Herold said. In fact, the Herolds’ research shows that children with COVID-19 have fewer neutralizing antibodies than adults. (Both kids and adults usually make enough antibodies to thwart future coronavirus infections after natural infection or vaccination.)
While the adaptive immune system can be effective, it can sometimes cause more harm than good.
Like soldiers who kill their comrades with friendly fire, a hyperactive immune system can cause collateral damage, triggering an inflammatory cascade that tramples not just viruses, but also healthy cells throughout the body.
In some COVID-19 patients, uncontrolled inflammation can lead to life-threatening blood clots and acute respiratory distress syndrome, which occurs when fluid builds up in the air sacs of the lung and makes it difficult to breathe, Betsy Herold said. Both are common causes of death in adult COVID-19 patients.
Because kids typically clear the coronavirus so quickly, they usually avoid this sort of dangerous inflammation, she said.
Research shows that healthy children have large supplies of a type of peacekeeper cell, called innate lymphoid cells, that help calm an overactive immune system and repair damage to the lungs, said Dr. Jeremy Luban, a professor at the University of Massachusetts Medical School.
Kids are born with lots of these cells, but their numbers decline with age. And both children and adults who are sick with COVID-19 tend to have fewer of these repair cells, Luban said.
Men also have fewer repair cells than women, which could help explain why males have a higher risk of dying from COVID-19 than females.
Both children and adults can develop “long COVID,” the lingering health issues experienced by about 10% of younger adults and up to 22% of those 70 and older. Studies suggest that 4% to 11% of kids have persistent symptoms.