As cases of rare COVID-linked illness in kids rise, Fauci warns much remains unknown

"The more and more we learn, we're seeing what this virus can do," Dr. Anthony Fauci said during a Senate hearing Tuesday.

The number of children with a rare and potentially deadly inflammatory condition likely linked to COVID-19 has risen to more than 100 in at least 14 states.

The tally comes as the Centers for Disease Control and Prevention says it's actively preparing guidance for health officials to track what's now being called pediatric multisystem inflammatory syndrome, or PMIS, nationwide.

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The vast majority of the cases are in New York, where Gov. Andrew Cuomo announced Tuesday the state has treated about 100 patients who range in age from younger than one to 18 years old.

At least five children in New York have died from the illness, which acts much like toxic shock syndrome or Kawasaki disease, causing severe inflammation of the coronary arteries.

"I know parents around the state are very concerned about this," Cuomo said during a media briefing Tuesday. "If we have this issue in New York, it's probably in other states."

Indeed, as awareness of PMIS has grown over the past few weeks, doctors have reported a small number of other cases in California, Connecticut, Delaware, Illinois, Kentucky, Louisiana, Massachusetts, Mississippi, New Jersey, Ohio, Pennsylvania, Utah, Washington, and Washington, D.C.

On Tuesday, a CDC spokesperson told NBC News the agency is working to come up with a standard definition of PMIS. The guidance is meant to assist public health officials in tracking cases in each state.

Identifying the syndrome can be difficult, doctors say, because many affected children don't exhibit the typical respiratory symptoms associated with the coronavirus.

Children are arriving at the hospital "with things like very high fever, severe abdominal pain and dropping blood pressure," said Dr. Roberta DeBiasi, head of the division of infectious diseases at Children's National Hospital in Washington, D.C.

DeBiasi said the children's hearts either aren't pumping normally or there is inflammation in the coronary arteries that supply blood flow to the heart. Some patients, but not all, also develop red eyes, rashes, swollen hands or enlarged lymph nodes.

And while many of the young PMIS patients have tested positive for COVID-19, or at least were found to have antibodies for the virus, not all children have. That may be because some kids don't appear to develop symptoms until a month after a COVID-19 infection.

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, warned a Senate committee Tuesday that much remains unknown about COVID-19 "particularly when it comes to children."

"We really have to be very careful," Fauci said, "because the more and more we learn, we're seeing what this virus can do."

Meanwhile, there is growing evidence that COVID-19 can and does affect children much more severely than previously thought, even if they're not ultimately diagnosed with PMIS.

DeBiasi and colleagues at Children's National Hospital analyzed medical records of 177 children and young adults in their 20s who tested positive for COVID-19. A quarter needed to be hospitalized, and a small percentage (5 percent) were critically ill and on life support.

Those most likely to require critical care interventions, such as mechanical ventilation, were either very young children, or youngsters over age 15. One of those patients was diagnosed with PMIS.

The research was published Tuesday in the Journal of Pediatrics. After the paper was submitted for publication, DeBiasi said the hospital treated about 70 additional children with the coronavirus, as well as two more with PMIS.

Those with underlying neurological conditions, such as cerebral palsy, have been significantly more likely to require hospitalization.

DeBiasi cautioned that coronavirus complications remain rare in kids, and that it's clear the vast majority of pediatric COVID-19 patients seem to have mild illnesses, or no symptoms at all.

But she said PMIS cases, so far, appear to be more prevalent on the East Coast of the U.S. and also in Europe, than had been reported in China and the U.S. West Coast.

"On the East Coast, and in particular speaking for the Washington, D.C., metropolitan region," DeBiasi said, "we're seeing more sick kids than we had expected."