Late last month, British health authorities warned the world that they were seeing a small but surprising number of children who had become critically ill with a mysterious inflammatory syndrome.

The symptoms of the new illness varied, although it had features of a few well-defined pediatric diseases. Only about half of children with the syndrome tested positive for coronavirus infection, but their sickness developed about three weeks after COVID-19 cases peaked in the United Kingdom.

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Now, hospitals, health departments, medical groups, and researchers worldwide are on alert for what has been awkwardly named “pediatric multi-system inflammatory syndrome temporally associated with COVID-19.″ It is the latest apparent complication of the insidious virus, which usually causes mild symptoms, or none at all, in children.

The new syndrome “is definitely its own thing,” said Audrey John, chief of infectious disease at Children’s Hospital of Philadelphia, which has treated a handful of children, who have recovered. “The association with COVID-19 is strongly suggestive but not definitive.”

At St. Christopher’s Hospital for Children, which has treated three children, who have recovered, epidemiologist and infectious disease specialist Ishminder Kaur echoed, “To see such a large cluster, it seems logical to link it to the coronavirus.”

Circumstantial evidence is mounting. A study conducted at the epicenter of Italy’s outbreak, published Wednesday evening in the Lancet, found that only 19 local children had been diagnosed with a rare inflammatory illness called Kawasaki disease in the five years before February, but there were 10 cases of Kawasaki-like illness between mid-February and April 20.

“Understanding this inflammatory phenomenon in children might provide vital information about immune responses” in children and adults, two British pediatricians wrote in an editorial accompanying the study.

» FAQ: Your coronavirus questions, answered.

Children with the new syndrome have a persistent high fever. Beyond that, their symptoms may fit all or part of the definitions of Kawasaki disease, which typically inflames blood vessels, or toxic shock syndrome, in which the blood pressure plunges so low that organs fail.

“Features observed included rash; conjunctivitis [pinkeye]; red, swollen hands; and red, cracked lips,” according to an article from Boston Children’s Hospital. “Some children had signs of cytokine storm syndrome,” an overreaction of the immune system that has been fatal in adult COVID-19 patients.

“Finally,” the article says, “many children had coagulopathies [clotting abnormalities], cardiac dysfunction, diarrhea ... and other GI symptoms.”

Although many children with the syndrome have tested negative for coronavirus infection, some of those children have tested positive for antibodies to the virus, a sign of immune response. “That suggests the inflammatory complications were delayed, occurring when the virus was no longer detectable,” the Boston Children’s article notes.

One heartening aspect of the newly recognized menace is that the outcomes are mostly good. Immune-modifying drugs, notably steroids and intravenous immunoglobulins, are very effective treatments.

Typically, “children have fevers stop that day," said John at CHOP. “Heart function tends to return to normal in a couple days.”

“Parents should be aware, but definitely not panic,” said Kaur at St. Chris.

The Pennsylvania Department of Health this week directed hospitals and health-care providers to report cases that fit the evolving definition of the inflammatory syndrome. Numbers are not yet publicly available.

“Cases should be reported regardless” of whether diagnostic coronavirus tests are positive, the department advised. The number to call is 877-724-3258.