300 children in the U.S. are known to have suffered from a rare inflammatory syndrome thought to be linked to the coronavirus which causes COVID-19, according to research.

Those diagnosed with the condition known as multisystem inflammatory syndrome in children (MIS-C) are sick enough to require hospital treatment; have a fever lasting for at least 24 hours; signs of inflammation in their body; and problems with multiple organs, according to the U.S. Centers for Disease Control and Prevention. Patients must also be 21-years-old or under, and either have tested positive for the COVID-19-causing coronavirus, had a positive antibody test, or been exposed to a COVID-19 patient four weeks before their MIS-C symptoms start.

Two papers published in the New England Journal of Medicine on Monday show at least 285 children have had MIS-C in the U.S. since March.In one study, researchers found 186 cases of MIS-C had been reported in 26 states between March 15 and May 20. The study excluded cases in New York State that were featured in the other study.

On average, the children were aged 8-years-old and hospitalized for seven days. A total of 62 percent were male, 73 percent were previously healthy, and 70 percent had either tested positive for the coronavirus after a swab or antibody test. Some 80 percent needed intensive care treatment, and a fifth were hooked up to a ventilator. The illness of 40 percent of patients had similarities to a rare condition called Kawasaki’s disease, characterized by inflammation in the coronary arteries and other blood vessels.

The team concluded that MIS-C in children linked with the coronavirus “led to serious and life-threatening illness in previously healthy children and adolescents.”

The study on children in New York featured 95 patients confirmed to have MIS-C in the state between March 1 and 10. In this group, 54 percent were male, and most children were aged between six and 12-years-old. Some 80 percent needed intensive care treatment, and two died. Most stayed in hospital for six days.