The potential explanations for the milder COVID-19 presentation in children in comparison to adults
The children have a lower percentage of the cases, and less severe illness courses. Some possible explanations, including the tendency to catch and occupy the upper airway. Also, the different expression of receptors of angiotensin-converting enzyme and renin–angiotensin system. A less vigorous immune response in children accounts for the less severity. In children, the lower levels of interleukin (IL)-6, IL-10, myeloperoxidase, and P-selectin and a higher intracellular adhesion molecule-1 can contribute to interpret the question under study. In children, higher level of lymphocytes and natural killers (NK) have a possible role in protection, and also infiltrations of the lung might have protective roles in the immunity. Elevated concentration of immunoglobulin-G antibodies against respiratory syncytial virus and mycoplasma pneumonia, may account for the protection against SARS-CoV-2 infection. A less vigorous adaptive system beside a powerful innate response and a healthy respiratory system, might protect children against SARS-CoV-2 infection. This paradox may be interpreted via the immunological considerations because the cytokine storm is thought to be important in the etiology of severe SARS-CoV-2 infections. Elevated inflammatory biomarkers are less common in children. In adults, there is a gradual decrease of naïve T cell count related to the thymic changes, wane in the collection of T cell clones generated in the thymus, and deprivation of costimulatory receptor CD28. All these reasons may deteriorate the immunological responses in adults. Protecting children against SARS-CoV-2 infection arises also through the trained immunity with cross-reactive neutralizing antibodies.
Ref: Why COVID‑19 is less frequent and severe in children: a narrative review. Reza Sinaei, Sara Pezeshki, Saeedeh Parvaresh and Roya Sinaei. (2020). World Journal of Pediatrics