Lower levels of ACE2 nasal gene expression in children may explain why children have a lower risk of Covid-19 infection and mortality. The SARS-CoV-2 virus uses ACE2 to enter the host. ACE2 nasal gene expression could potentially be used as a biomarker to evaluate Covid-19 susceptibility.

A retrospective analysis of 305 patients aged 4 to 60 years encountered within the Mount Sinai Health System in New York. ACE2 gene expression in nasal epithelium, the first point of contact for SARS-CoV-2 and the human body, was lowest in younger children and increased with age.

These findings could help explain why children appear to be less susceptible to Covid-19 infection. The results may point to a potential biomarker of Covid-19 susceptibility. Prospective studies are needed to assess the degree to which ACE2 expression can be used as a biomarker for COVID-19 susceptibility.

"Why children get COVID-19 less than adults has been a puzzle. Researchers have hypothesized that lower expression of ACE2, which the SARS-Cov-2 virus uses to enter our bodies, might explain why children are less likely to get COVID-19. Our study shows that ACE2 expression in the nasal epithelium is lowest in younger children and increases with age into adulthood. Our results may help explain why children account for less than 2% of identified cases of Covid-19. A biomarker of COVID-19 susceptibility based on ACE2 expression might be possible," said  the researcher.

https://www.mountsinai.org/about/newsroom/2020/mount-sinai-researchers-why-covid19-may-be-less-common-in-children-than-adults

https://jamanetwork.com/journals/jama/fullarticle/2766524