But the virus is the same, whether it infects adult or child. The question is, why does COVID-19 affect them so differently? Both striking patterns in kids—the fact that most do not get very sick but a small number still end up with a delayed inflammation syndrome—may be rooted in a child’s still-developing immune system. And while COVID-19 is a new disease, these patterns are seen with other viruses too.
Immune systems change with age, becoming weaker or stronger in different ways. An adult’s body might be better armed against familiar threats, but more inflexible against novel ones. The two human viruses most closely related to the coronavirus that causes COVID-19 are the ones behind SARS and MERS—both also coronaviruses, a large family that infects many animal species. SARS likely jumped from bats to civet cats to humans in 2002, and MERS from camels to humans in 2012. Both have a much higher fatality rate than COVID-19 and neither exploded into a pandemic on the current scale, giving us smaller numbers from which to draw conclusions. Still, they too seemed to have largely spared children.
Like COVID-19, SARS and MERS were caused by viruses entirely new to humans, and adult immune systems are unused to dealing with entirely new viruses. By and large, the ones that sicken adults year after year are altered versions of viruses they’ve encountered before, like seasonal flu. Children, on the other hand, are constantly dealing with viruses that are not necessarily novel but are novel to them. “Everything an infant sees, or a young child sees, is new,” says Donna Farber, an immunologist at Columbia University. Thus, their immune system is primed to fight new pathogens in a number of ways.
Babies are born, for example, with a complete repertoire of immune cells called T cells. Every T cell has a unique receptor, and taken together, the pool of millions of T cells can recognize virtually any hypothetical pathogen. As the child begins encountering pathogens, though, their immune system winnows this diverse repertoire. It keeps the T cells involved in fighting off pathogens as a pre-stocked arsenal of “memory T cells,” should those pathogens appear again, but begins losing the others. This is why adults are able to mount a rapid immune response to previously encountered pathogens but also why they might have trouble fighting a new one. Diseases such as rubella and chicken pox are also, for various reasons, more severe in adults than in children. The pattern with seasonal flu is different, Farber says, but that may be because immunity against previous strains of the flu offer some crossover protection in adults.
The same may actually be true for coronaviruses, too, only in children. Another hypothesis for why most kids are spared is that they are frequently infected with the four coronaviruses that cause some common colds. These cold coronaviruses are not as closely related to COVID-19 as SARS or MERS, but they still share some similarities. Immunity against these cold coronaviruses wanes over time, so children who have been recently exposed might have some protection that adults don’t.