Much has been made of the seeming evidence that children who contract the Covid-19 coronavirus rarely develop obvious symptoms of the disease.

But if that’s the rule, it’s now apparent that there are exceptions to it.

Health authorities in New York City say that 15 children were hospitalized with acute symptoms possibly related to the virus outbreak in the two weeks from April 17 to May 1.

The NYC Health Department describes the illness as “a pediatric multi-system inflammatory syndrome,” similar to toxic shock syndrome or Kawasaki disease, a rare childhood illness that causes inflammation in blood vessels.

The symptoms included fever, rash, abdominal pain, vomiting, or diarrhea, but fewer than half the young patients suffered respiratory problems.

“The syndrome has received growing attention in recent weeks as similar cases have begun appearing in European countries hit hard by the coronavirus,” including Britain, France, Italy and Spain, reports the New York Times.

Most of the patients either tested positive for the coronavirus or were found to have antibodies for Covid-19 in their bloodstreams.

None of these latest New York patients have died, although six other young people in the city, aged 17 and under, have succumbed to the disease. The total number of Covid-19-related deaths in the city is approaching 16,000.

Dr. Oxiris Barbot, the NYC Health Commissioner, issued a statement alerting doctors and other health providers about what she called a “recently recognized syndrome in children so that they can be diagnosed and treated early to avoid long-term complications.”

She added: “And to parents, if your child has symptoms like fever, rash, abdominal pain or vomiting, call your doctor right away.”

Even so, the Washington Post reports, Dr. Jeffrey Burns, chief of critical care at Boston’s esteemed Children’s Hospital, has some reassuring words:

It would be a terrible mistake for parents to worry,” he said. “The chance for a child up to this point of becoming critically ill is very, very rare.”