O'FALLON, Mo. -- Medical researchers in St. Louis are trying to determine why a small percentage of children and young adults who test positive for COVID-19 develop a life-threatening condition that can cause severe inflammation of the heart, brain and other organs, university officials said Friday.

Washington University pediatric specialists are part of a research team led by the National Institutes of Health investigating how the coronavirus affects children and young adults up to age 21, who account for about 13% of infections in the U.S.

Most children and young people survive the virus without serious illness, with many showing no symptoms at all. But researchers at Washington University want to know why some of them, weeks after infection, develop a condition called Multisystem Inflammatory Syndrome in Children, or MIS-C, which causes inflammation of the heart, lungs, kidneys, brain, skin, eyes and gastrointestinal organs. Most children with the syndrome survive, but researchers say the long-term effects are unknown.

Black and Hispanic children are more likely to get MIS-C, according to researchers.

"While MIS-C is rare, it is worrisome because most children hospitalized with the condition were healthy just a few weeks before," Dr. Charles Canter, a Washington University professor of pediatrics who is leading the investigation, said in a news release. "We can't predict which children will develop it and why. The aim of this study is to understand what drives the development of MIS-C and severe COVID-19 in children so we can identify those who are at risk and determine the best way to treat them."

The Missouri Department of Health and Senior Services on Friday cited 430 newly confirmed cases of COVID-19 and eight new deaths from the disease, pushing its pandemic totals to 479,966 confirmed cases and 8,158 deaths.