More Clues to Why Kids Have Much Milder COVID-19

TUESDAY, Dec. 8, 2020 (HealthDay News) -- A stronger immune system and healthier blood vessels are among reasons kids are less likely than adults to have severe COVID-19, according to experts who reviewed research from around the world.

"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. Even children with the usual risk factors for severe infections, such as immunosuppression, were not at high risk of severe COVID-19 disease," said review co-author Nigel Curtis. He's a professor of pediatric infectious diseases at the Murdoch Children's Research Institute in Melbourne, Australia.

Learning more about how age affects the risk of severe COVID-19 could improve prevention and treatment, Curtis said in an institute news release.

The researchers explained that damage to the thin layer of endothelial cells lining various organs -- especially the heart, blood vessels and lymphatic vessels -- increases with age, and there's a link between conditions that damage 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," Curtis said. "COVID-19 can infect these endothelial cells and cause blood vessel inflammation."

Compared to adults, children's endothelium has experienced far less damage and they are less prone to abnormal blood clotting, Curtis added.

Diabetes, obesity and other aging-related diseases associated with chronic inflammation are also connected with severe COVID-19, according to the authors of the review published online Dec. 1 in the Archives of Disease in Childhood.

In addition, recent immunization with live vaccines that boost the immune system (such as the childhood shots against measles, mumps and rubella) might help protect kids against severe COVID-19, the study authors said.

There are other significant differences between child and adult immune systems, said co-author Dr. Petra Zimmermann, a senior lecturer at the University of Fribourg in Switzerland.

"Children have a stronger innate immune response, which is the first-line defense 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.

Kids infected with COVID-19 are often infected with other viruses as well, she pointed out. "Recurrent viral infections could lead to improved trained immunity, making kids more effective at clearing COVID-19," Zimmermann said.

Different levels of bacteria and other microbes in the throat, nose, lung and stomach also affect susceptibility to COVID-19.

"The microbiota plays an important role in the regulation of immunity, inflammation and in the defense against illnesses," Zimmermann added. "Children are more likely to have viruses and bacteria, especially in the nose, where these bugs might limit the growth of COVID-19."

Levels of vitamin D, an anti-inflammatory, also tended to be higher in children, according to the review.

"The overlap between risk factors for severe COVID-19 and vitamin D deficiency, including obesity, chronic kidney disease and being of Black or Asian origin, suggests that vitamin D supplementation may play a role in helping prevent or treat COVID-19," Zimmermann said.

In many countries, infants and toddlers are routinely given vitamin D supplements, she added.

More information

The U.S. Centers for Disease Control and Prevention has more on children/teens and COVID-19.

SOURCE: Murdoch Children's Research Institute, news release, Dec. 2, 2020