COVID-19 was thought to have little effect on children. But according to doctors in New York City, 15 children have been hospitalized for a rare but serious disorder after contracting the coronavirus.

The New York City Department of Health issuing a warning for doctors to be on the lookout for this serious syndrome, first reported lat month in Europe.

Doctors say the children first showed symptoms including fever, rash and stomach illness, similar to a rare condition called Kawasaki Disease, which causes the inflammation of blood vessels and can lead to heart problems.

As a result, The World Health Organization is investigating whether the coronavirus causes some children to develop a rare inflammatory disease, WHO officials said Wednesday.

Health officials in the U.K. warned doctors over the weekend that Covid-19 could be causing a rare inflammatory condition in children. Britain’s Pediatric Intensive Care Society said Monday the National Health Service alerted it to a small number of critically ill children presenting with “an unusual clinical picture.”

The society noted that many of the children with symptoms of the new inflammatory disease had been diagnosed with Covid-19. The condition was likened to toxic shock syndrome and Kawasaki disease.

“We are aware of this report which came out of the United Kingdom about a small number of cases amongst children with this inflammatory response,” WHO’s lead scientist on Covid-19, Dr. Maria Van Kerkhove, said Wednesday. “We’re looking at this with our clinical network.”

It remains unclear what the relationship is between Covid-19 and the inflammatory conditions, but Chris Whitty, England’s chief medical officer, told reporters Monday it is “entirely plausible” that Covid-19 causes the illness.

“There are some recent rare descriptions of children in some European countries that have had this inflammatory syndrome, which is similar to the Kawasaki syndrome,” Van Kerkhove said. “But it seems to be very rare.”