New York, Jul 9 (IANS): Differences in lung physiology and immune function in children could be why they are more often spared from severe illness associated with COVID-19 than adults, say researchers.

According to the study, published in the journal 'American Journal of Physiology - Lung Cellular and Molecular Physiology', only about 1.7 per cent of the first 149,082 cases in the US were infants, children, and adolescents younger than 18 years old.

The researchers noted that children under 18 make up 22 per cent of the US population and only three pediatric deaths were identified by the Centers for Disease Control and Prevention (CDC) as of April 2020.

Angiotensin-converting enzyme 2s, called ACE2, are the doors that allow SARS-CoV-2, the novel coronavirus that causes COVID-19, to enter the body's cells. Children naturally have less ACE2 in the lungs than adults, the study said.

"ACE2 are important for viral entry and there seems to be less of them in children because they increase with age," said study senior author Matthew Harting from the University of Texas Health Science Center at Houston (UTHealth) in the US.

In addition to fewer ACE2 receptors, the authors noted the immune system in children responds to viruses differently than that of adults, leaving less opportunity for severe illness in pediatric patients.

There are several different mechanisms behind the differences, including the retention of T-cells in children, which are able to fight off or limit inflammation.

"T-cells have a viral response and also an immune modulator response. In severe cases of adult COVID-19 patients, we've seen that those T-cells are reduced, so the ability to fight the virus is also reduced," said study co-author Harry Karmouty-Quintana.

"In kids, those T-cells seem to be maintained, so they are still able to prevent the virus," Karmouty-Quintana added.

Lung tissue in children naturally has a higher concentration of regulator T-cells. Patients with higher levels of T-cells also have higher levels of Interleukin 10 (IL-10), also known as human cytokine synthesis inhibitory factor, an anti-inflammatory cytokine.

"IL-10 inhibits the inflammation of other components like IL-6 that are detrimental. Adults tend to experience hyperinflammatory state, while kids do not," Karmouty-Quintana informed.

"In preclinical studies in mice, IL-10 has also shown to decrease with age," the study authors wrote.

Recently, a 26-country review compiled from 131 studies, the largest systematic review to date of children and young adults with Covid-19, found that the majority of children with Covid-19 fared well clinically compared to adults during the first four months of the pandemic.