One of the most troubled questions of the pandemic is why the symptoms of COVID-19 tend to be less lethal in children and young adults than in older people. A new study suggests that the immune systems of people younger than 24 obstructs the coronavirus with a powerful first wave. The defence mechanisms of the body to attack the intruder entering the system could be weaker in older people.

Research shows that younger people generally get less sick. It’s possible when compared with adults and older kids, younger children have a lower amount of ACE2 protein in their upper respiratory tracts. That’s the protein that the virus uses to break into the cells. Another explanation is that younger people have less virus in their bodies, which could mean milder symptoms, although studies have shown that viral load is similar across people no matter their age, differences in the immune system, which tends to become less robust with age, could play a role.

When compared with adults who had severe COVID-19, younger patients who did not need a ventilator also had lower levels of other immune signalling proteins involved in inflammation. Overreaction is responsible for severe symptoms in many of the sickest COVID-19 patients. Some adults, especially those on a ventilator or who had died, had a stronger response of T cells that recognized a protein on the virus’ surface, called the spike, compared with younger people. People older than 24 also made more antibodies that prevent the virus from infecting new cells, called neutralizing antibodies. In the study, a high T cell response wasn’t associated with mild disease, hinting that those responses weren’t necessarily protective.