At least 15 children in New York City hospitals have been found to have symptoms related to a rare illness called Kawasaki disease, raising concerns over a possible association with the new coronavirus.

Also called mucocutaneous lymph node syndrome, Kawasaki disease causes inflammation in the walls of some blood vessels.

The patients, between the ages of 2 and 15, were found to have the symptoms of either Kawasaki disease or toxic shock syndrome, including a persistent fever, according to the NYC health officials.

They were found to have the symptoms between April 29 and May 3. All the patients had a fever, while more than half of them had complaints of a skin rash, vomiting, abdominal pain, or diarrhea. And less than half of the patients had respiratory symptoms. Among them, four patients tested positive for COVID-19, while 11 tested negative.

The NYC health department said no deaths have been reported. However, most patients required blood pressure support, while some required mechanical ventilation.

Some experts suspect that there is an association between Kawasaki disease and COVID-19, while others believe that there is no connection at all.

Dr. Frank Esper of the Cleveland Clinic’s Center for Pediatric Infectious Diseases said, “Kawasaki disease is one of the great mysteries in pediatrics. It’s something we’ve been dealing with for decades.”

Doctors know how to treat Kawasaki disease but they still do not know what its causes are and why children are affected.

“Kawasaki disease is the boogeyman to pediatricians,” Dr. Esper said. “It’s extremely difficult to diagnose. Even with the most astute clinicians, we have a hard time figuring out who has it and who doesn’t.”

Dr. Esper explained that the main indicator of the disease could be found in the heart, causing coronary artery aneurysm, which distinguishes Kawasaki disease from other inflammatory diseases.

Is there a connection between Kawasaki disease and COVID-19?

Some experts say that it is too early to tell whether Kawasaki is associated with COVID-19.

“We’ve never seen the coronavirus before but we’ve been dealing with Kawasaki disease for decades,” Dr. Esper said.

He noted that experts are not sure whether the mystery diseases that popped up in NYC can be identified as Kawasaki disease. He also said he has not seen any reports that mention coronary artery dilatation, which is the major indication of Kawasaki.

“I will caution that there are many things that look similar to Kawasaki disease,” Dr. Esper said. “It could be that what they’re calling Kawasaki is not Kawasaki but an inflammatory disease caused by the coronavirus.”

Dr. Sunil Sood of Northwell Health’s Southside Hospital and Cohen Children’s Medical Center said he does not believe the condition is Kawasaki.

Although the NYC Health Department only mentioned 15 cases, Dr. Sood said he has seen at least 20 such cases between the two hospitals he is affiliated with. He said only three of his cases have been tested positive for COVID-19, but the rest tested positive for coronavirus antibodies.

“The immune system can overreact in a delayed timeline many weeks later,” Dr. Sood said. “We know this from other infectious diseases.”

Dr. Sood has urged parents to take their kids to the hospital if they develop any symptoms of Kawasaki disease because it could increase the risk of heart complications.

“Initially, I thought it was Kawasaki … but it’s going beyond those symptoms,” he added. “Pediatricians and parents should be aware that there’s an outbreak of this right now.”