At least 15 children in New York City have been hospitalized with a mysterious illness believed to be linked to COVID-19.
The children, who range in age from 2 to 15 years old, have shown symptoms consistent with other inflammatory illnesses, such as Kawasaki disease and toxic shock-like syndrome, which affect the heart and blood vessels.
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Other cases of what's now being called "pediatric multisystem inflammatory syndrome" have also been reported in Europe.
Some, but not all, affected children have tested positive for the coronavirus. A few have tested negative, though they had been exposed to infected family members.
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"We are still working out an understanding" of the syndrome, said Dr. Jane Newburger, director of the Kawasaki Program at Boston Children's Hospital. "The concern is that this is probably, at least for some of them, a post-immune reaction to COVID."
The New York City Department of Health and Mental Hygiene issued an alert about the pediatric cases to physicians Monday.
"Clinical features vary, depending on the affected organ system, but have been noted to include features of Kawasaki disease or features of shock; however, the full spectrum of disease is not yet known," Dr. Demetre Daskalakis, deputy commissioner for the department, wrote.
Children have had fevers and elevated levels of inflammation throughout the body. Some have been admitted to pediatric intensive care units. No deaths have been reported.
Newburger said there are surely more cases than the 15 patients reported in New York City.
"This is happening all through Europe," she told NBC News. "It is definitely happening in various cities on the East Coast and in some parts of the Midwest."
While the condition requires much more study, it appears to be rare. Indeed, doctors say most kids with COVID-19 have no or mild symptoms, and are usually spared most serious complications.
"However, children with underlying health conditions who develop COVID-19 may be more at risk for having more severe illness," Jonah Bruno, director of communications for the New York State Department of Health, said in a statement.
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"This highlights the importance of efforts to support our scientific community performing research for therapies and a vaccine, while we also continue social distancing measures to decrease the spread of disease," he said.
Though this new syndrome appears to be rare, if parents find that their child develops a high fever and "seems unwell," Newburger said, "they definitely should call their pediatrician and seek medical attention."
Meanwhile, the National Institutes of Health is launching a study to determine exactly how many children in the U.S. have been infected with COVID-19, and if so, how the disease manifests.
The project aims to recruit 6,000 people from 2,000 families in 11 cities across the country. The participants will be tracked for six months.