The receptor ACE2, which allows the virus entry at the level of the nasal epithelium, is much less present in children than in adults, which could explain the low incidence of infection affecting more young people.
One of the main characteristics of the pandemic Covid-19 current is without a doubt how this disease affects disproportionately the most vulnerable populations, especially the elderly and those affected by pre-existing conditions (obesity, diabetes, cardiovascular and lung diseases, cancer).
Children little affected
The children, however, are very little affected by the disease .
In the United States, for example, of the 149 082 case of Covid-19 listed date of 2 April 2020, only 2572 involved persons under 18 years of age (1). Children are, therefore, less than 2% of all cases of Covid-19 in the american population, which corresponds to the current situation in Quebec and in several other places in the world.
In children, the symptoms of infection by the coronavirus are less frequent and less severe than in adults. Since tests to detect the Covid-19 are mostly reserved for people who have symptoms, it is therefore possible that the lower incidence of Covid-19 measured in young simply reflects a lower number of tests carried out with this population.
By contrast, studies where the detection tests were performed in a random way (without being limited to symptomatic individuals) suggest that the lower incidence of Covid-19 among young people is rather linked to a greater resistance of this population to the virus. In Iceland, for example, while approximately 1 % of the population of 364 000 inhabitants has been reported in a positive coronavirus result of such random testing, no cases were detected in children less than 10 years (2).
From the beginning of the pandemic, researchers have shown that the coronavirus SARS-CoV-2 interacts with the protein surface ACE2 in human cells, the latter acting as a receptor that allows the virus to penetrate to the interior of the cells. This receptor ACE2 is preferentially localized in three cell types are distinct, is the pneumocyte type II cells (at the level of the pulmonary alveoli), the enterocytes of the gut and the secretory cells of the mucosa (epithelium) of the nasal cavity (3). In the latter case, the results pre-publication suggest that the infection level of the cells of the nasal wall generates inflammation which disrupts the nerves present in the vicinity, which would explain the loss of sense of smell (anosmia) reported by a significant proportion of people infected by the sars coronavirus (4).
A study recently published in the journal of the american medical association (JAMA) suggests that the lower susceptibility of children to infection by the coronavirus could be due to a reduction in the number of receptor ACE2 at the level of the nasal epithelium (5). Researchers have assessed the expression of the gene encoding for this receptor in samples of epithelium in the nasal samples taken from patients aged 4 to 60 years. They observed that the youngest children (under 10 years old) expressed the lowest amounts of ACE2 and that the number of receptors increased later gradually with the age. Since it is well documented that the nose is an important gateway to the virus, it is therefore possible that the lower incidence of Covid-19 observed in children is simply due to an inability of the coronavirus to bind effectively to the surface of the cells of their respiratory system.
♦ (1) CDC COVID-19 Response Team. Coronavirus disease 2019 in children—United States, February 12–April 2, 2020. MMWR Morb Mortal Wkly Rep. 2020; 69: 422-426.
♦ (2) Gudbjartsson DF et al. Spread of SARS-CoV-2 in the Icelandic population. N. Engl. J. Med., published on April 14, 2020.
♦ (3) Ziegler CGK et al. SARS-CoV-2 receptor ACE2 is an interferon-stimulated gene in human airway epithelial cells and is detected in specific cell subsets across tissues. Cell, published April 27, 2020
♦ (4) Brann DH et al. Non-neuronal expression of SARS-CoV-2 entry genes in the olfactory system suggests mechanisms underlying COVID-19-associated anosmia. bioRxiv, published on April 9, 2020.
♦ (5) Bunyavanich S et al. Nasal gene expression of angiotensin-converting enzyme 2 in children and adults. JAMA, published may 20, 2020.