Nearly 300 cases of a rare, life-threatening syndrome in children and adolescents associated with the novel coronavirus have been identified in the United States in two studies in The New England Journal of Medicine.

The US studies published on Monday (June 29) follow several reports of the syndrome among Covid-19 patients in France, Italy, Spain and Britain.

Multisystem Inflammatory Syndrome in Children (MIS-C), shares symptoms with toxic shock and Kawasaki disease, including fever, rashes, swollen glands and, in severe cases, heart inflammation.

A consistent picture is emerging of the syndrome occurring two to four weeks after infection by the coronavirus, Michael Levin, professor of paediatrics and international child health at Imperial College London, said in an accompanying editorial.

The syndrome affects 2 in 100,000 young people, defined as under age 21, out of 322 in 100,000 in that group who get Covid-19, he wrote.

While the studies identified about 300 cases in the US, Levin noted that there have been more than 1,000 cases reported worldwide and that a relatively high proportion have occurred among Black, Hispanic or South Asian persons.

"There is a concern that children meeting current diagnostic criteria for MIS-C are the 'tip of the iceberg' and a bigger problem may be lurking below the waterline," Levin wrote.

The first study, led by Boston Children's Hospital, found 186 cases of MIS-C in 26 US states, with 4 out of 5 cases needing intensive care and one out of five requiring mechanical ventilation. Four patients died.

The second study, which observed patients in New York and was conducted by the state's health department, found another 95 confirmed cases, with 4 out of 5 needing admission to intensive care unit and two patients dying.

It is not clear why MIS-C develops in some children and adolescents and not in others.

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