In the study, published in the journal Archives of Disease in Childhood, the research team explained why respiratory viruses, severe disease and death due to Covid-19 was relatively rare in children.
"Most children with Covid-19 have no or only mild symptoms, most commonly fever, cough, sore throat and changes in sense of smell or taste," said study author Nigel Curtis from the University of Melbourne in Australia.
"Even children with the usual risk factors for severe infections, such as immunosuppression, were not at high risk of severe Covid-19 disease," Curtis added.
The researchers said that damage to the thin layer of endothelial cells lining various organs, especially the blood vessels, heart, and lymphatic vessels, increased with age and there was an association between conditions that affect these cells and severe Covid-19.
"We know pre-existing blood vessel damage plays an important role in Covid-19 severity and can lead to blood clots, causing strokes and heart attacks. Covid-19 can infect these endothelial cells and cause blood vessel inflammation," Curtis said.
"The endothelium in children has experienced far less damage compared with adults and their clotting system is also different, which makes children less prone to abnormal blood clotting," he added.
The researchers said diseases associated with chronic inflammation that develops with advanced age including diabetes and obesity were also linked with severe Covid-19.
They said more recent immunisation with live vaccines, such as the MMR vaccine against measles, mumps, and rubella, that could boost the immune system might play a role in protecting children.
Dr Petra Zimmermann from the University of Fribourg said there were also other important differences in the immune system between children and adults.
"Children have a stronger innate immune response, which is the first-line defence against Covid-19," she said.
"Another important factor is 'trained immunity' which primes innate immune cells after mild infections and vaccinations, leading to a type of 'innate immune memory'," Zimmermann added.
According to the researchers, children infected with Covid-19 often have co-infections with other viruses. Recurrent viral infections could lead to improved trained immunity, making kids more effective at clearing Covid-19.