The city's Department of Health is reporting that 15 children, many of whom tested positive for COVID-19, have been hospitalized in NYC with a new illness that has also been seen in the United Kingdom.

The children, ranging in ages from 2 to 15, were hospitalized between April 17th to May 1st, 2020 with a "multi-system inflammatory syndrome" that health authorities said was compatible with Kawasaki disease, a treatable disease typically affecting young children that causes fever and inflammation of the blood vessels. It is described as the most common cause of acquired heart disease in children.

The 15 patients in New York City have been admitted to pediatric intensive care units. Five of the children are on ventilators, while others have require cardiac support. None have died.

Of the 15 children, four tested positive as currently having COVID-19, while 11 were negative. However, six of the children who tested negative in diagnostic testing were positive for antibodies.

"We’re not sure what to make of this yet," said Dr. Oxiris Barbot, the city's health commissioner during a press conference.

Barbot described Kawasaki disease as rare but that she had come across a case as a pediatrician. The symptoms include prolonged high fever, very red eyes, brightly red lips, and a swollen or "strawberry" tongue.

Treatment for Kawasaki includes immunoglobulin and aspirin, Barbot said.

In addition to the cases reported in the U.K, Barbot said there have also been a small number of cases in Philadelphia and Boston.

On April 19, the Boston Globe reported that 13 children had been hospitalized at Boston Children’s Hospital for suspected cases of COVID-19. At least three were admitted into intensive care. There was no mention, however, of Kawasaki disease.

News of the concerning new disease in New York City came in a health bulletin from the city's Health Department on Monday night. The bulletin goes out to health care providers as well as subscribers who have been instructed to contact the Department of Health if they see patients with similar symptoms.

In late April, 8-year-old Jayden Hardowar, a healthy Queens boy with no underlying conditions, developed a fever and diarrhea. NBC NY reports that his pediatrician prescribed Tylenol and after a few days his temperature came down, but his strength did not return. On April 29th, his mother heard him calling to her from his bed at night in "a very low" voice. From NBC:

The mother said she moved closer into the bed, and noticed her child's head and hands were twisted in an unorthodox position backward. "I quickly looked over at his face and his lips were all blue at that point, so right away I knew something was not right here with Jayden," Navita said. She started yelling his name, but he was not responding. Struggling to find a pulse, Roup and Jayden's older brother Tyrone — a 15-year-old Boy Scout — began performing CPR as Navita called 911.

Jayden was transferred to Cohen Children's Hospital in Nassau County, where he was put on a ventilator in the ICU. The Long Island hospital had already reported 40 similar pediatric patients, according to an earlier NBC report.

An antibody test at the hospital indicated that Jayden had contracted COVID-19 at an earlier, undetermined date. He is gradually recovering and was taken off the ventilator over the weekend -- his parents say they hope to take him home soon.

Health Commissioner Dr. Oxiris Barbot said the city was alerting thousands of providers throughout the city about the newly identified illness and symptoms.

"Even though the relationship of this syndrome to COVID-19 is not yet defined and not all of these cases have tested positive for COVID-19 by either DNA test or serology, the clinical nature of this virus is such that we are asking all providers to contact us immediately if they see patients who meet the criteria we’ve outlined," she said in a statement.

She added that parents whose children show symptoms like fever, rash, abdominal pain or vomiting should call their doctor right away.

Mayor Bill de Blasio spoke about the children and their symptoms during his press conference on Tuesday. “This is something that is causing concern and I want to be sure all New Yorkers are aware,” de Blasio said.

In the U.K., at least a dozen children have fallen ill with similar symptoms, all of them requiring intensive care. Most of those children were diagnosed with overlapping symptoms of toxic shock syndrome and Kawasaki disease. Like the U.S. cases, some tested positive for coronavirus, while some did not.

According to early studies, coronavirus does not appear to affect children as severely as adults, but it is unclear why.