Why is the coronavirus infection usually mild in children? Here’s a new clue - The Philadelphia Inquirer

Then they measured the activity of a gene that directs production of ACE2, a protein that helps coronavirus enter the body.

It turned out that ACE2 gene “expression” — the DNA instructions that are converted into a functional molecule — was lower in young children, and that expression increased with age.

"The results from this study show age-dependent expression of ACE2 in nasal epithelium, the first point of contact for [the coronavirus] and the human body.

The reduced ACE2 gene expression in children suggests, but doesn’t prove, that children create less of the protein that the virus uses as its gateway, or receptor.

To see whether the ACE2 gene may play a role, the Mount Sinai team used nasal lining samples that were provided by 305 people for a study conducted between 2015 and 2018.

ACE2 gene activity was lowest in children under 10, and rose steadily and significantly with age.

Although the researchers, led by pediatrician Supinda Bunyavanich, found ACE2 gene expression was linked to age, they noted that a study conducted before the pandemic of patients with severe respiratory distress found no link between ACE2 protein activity and age.

However, that study didn’t examine gene expression, and “the lung and nasal environments are distinct, with known differences in gene expression.".