This retrospective study show age-dependent expression of ACE2 in nasal epithelium, the first point of contact for SARS-CoV-2 and the human body. This may shed light on why children only appear to account for a small proportion of COVID-19 cases. 

• Angiotensin-converting enzyme 2 (ACE2) is the receptor that severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) uses for host entry. ACE2 gene expression was the focus of this study.
• Study examined the nasal epithelium from individuals aged 4 to 60 years encountered within the Mount Sinai Health System, New York, New York, during 2015-2018.
• Samples were collected from individuals with and without asthma for research on nasal biomarkers of asthma.
• 305 individuals aged 4 to 60 years was balanced with regard to sex (48.9% male). Because the cohort had been recruited to study biomarkers of asthma, 49.8% had asthma.
• ACE2 gene expression was lowest (mean log₂ counts per million, 2.40; 95% CI, 2.07-2.72) in younger children (n = 45) and increased with age, with mean log₂ counts per million of 2.77 (95% CI, 2.64-2.90) for older children (n = 185), 3.02 (95% CI, 2.78-3.26) for young adults (n = 46), and 3.09 (95% CI, 2.83-3.35) for adults (n = 29).
• A linear regression model adjusted for sex and asthma was built that also showed significant adjusted associations (P ≤ .05) between ACE2 expression and age group.
• Limitations: Did not include individuals older than 60 years. Tissue included was only from the nasal epithelium, and does not reflect ACE2 expression in the pulmonary epithelium (which is under different regulation).