Throughout the nearly five months the world has been fighting covid-19, one of the most comforting — and baffling — aspects of the coronavirus has been its effect on children. Most children who are infected with the coronavirus remain asymptomatic or experience mild illness. But there’s new evidence the disease may be associated with surprising complications in a small number of children.

Britain’s national health authority Monday issued an alert to physicians warning of an “apparent rise in the number of children of all ages presenting with a multi-system inflammatory state requiring intensive care across London and also in other regions” that may be related to covid-19, the disease caused by the coronavirus.

The National Health Service said the cases, seen during the past three weeks, have “common overlapping features” of toxic shock syndrome and another condition that results in dangerous blood vessel inflammation.

The description of symptoms potentially could be related to blood clotting complications described in a small number of critically ill adults diagnosed with covid-19.

British health authorities have declined to provide details, including the number of pediatric cases, other than to say serious complications related to the virus are “very rare” but a growing concern.

Simon Kenny, the National Health Service clinical director for children and young people, said in a statement emailed to The Washington Post the alert was issued because “it is important that clinicians are made aware of any potential emerging links so that they are able to give children and young people the right care fast.”

While there are no approved treatments that target the virus itself, physicians can provide oxygen, blood thinners, antibiotics and other supportive care that can help children avoid complications and keep them alive while their bodies clear the virus on their own. Moreover, the alert serves as a warning to physicians that those children may be infectious.

The Pediatric Intensive Care Society of the U.K. said in a statement Monday its members received the alert about “a small rise” in the number of cases of children presenting with “an unusual clinical picture” over the weekend. Some of the children had tested positive for covid-19 while others had not.

Those potential complications included toxic shock syndrome, a rare and potentially fatal condition that involves the release of toxins from a bacterial infection. It’s often talked about in the context of tampon use and women who are menstruating. Kawasaki disease, which affects mostly children under age 5, is characterized by an inflammation of blood vessels throughout the body that can lead to heart attacks or aneurysms.

The U.K. National Health Service said abdominal pain and gastrointestinal symptoms have been “a common feature” in these rare cases, along with cardiac inflammation. The cases have involved “blood parameters consistent with severe COVID-19 in children,” the alert stressed, but it is possible there may be a different “unidentified infectious pathogen” associated with these cases.

The Pediatric Intensive Care Society said while it’s “too early to say with confidence,” children in these cases appear to have irregularly high readings of three blood markers of inflammation.

After the alert was issued, doctors from Italy, Spain and other parts of the world tweeted they had seen similar cases.

Doctors at prominent pediatric hospitals in U.S. hot zones from Seattle to New York said they had not observed alarming increases in the number of pediatric patients being hospitalized for covid-19. Instead, physicians said they felt that increase correlated with a rise in cases in the general population, rather than an emerging threat to children.

Michael Agus, chief of medical critical care at Boston Children’s Hospital, said last week he had observed a “steady flow” of positive tests in children that “seems like it’s correlating with the peak for Massachusetts that the adults are seeing as well.”

On Monday, Janet A. Englund, an infectious diseases expert with Seattle Children’s Hospital, said she is not seeing “a large volume of pediatric patients admitted to our hospital or even being diagnosed with COVID-19 in Seattle at this time.” Monica Rizzo, a spokeswoman for Children’s Hospital Los Angeles, said one covid-19 patient who was treated there presented somewhat elevated levels of inflammation but nothing that constituted a pattern.

Children’s National Hospital in the District said it had treated 105 children with the coronavirus between March 15 and Wednesday. Roughly 40 percent of the children had an underlying condition. Twenty-eight were sick enough to require hospitalization. There have been no deaths.

Michael J. Bell, chief of critical care at Children’s National, said the hospital had not seen cases similar to those described in the National Health Service alert. He said there were two main types of patients in the ICU: infants and teens. In the infants, the coronavirus has acted similar to many other viruses that hit each year. In the older children, it looks more like acute respiratory distress syndrome — known as ARDS — which is a presentation common in adults.

But Bell said he has not seen other complications common in adults, such as uncontrollable inflammation or blood clots.

“Maybe we haven’t seen enough patients, or maybe younger people are protected,” he said. “Obviously, we are on the lookout.”

As of Saturday, the Centers for Disease Control and Prevention reported 12,791 confirmed cases of covid-19 in children under age 18.

Three children under the age of 15 have died in the United States, and 22 people between the ages of 15 and 24 have died.

One was the infant daughter of a New York firefighter. Jay-Natalie La Santa was 5 months old when she was hospitalized for treatment. The family has said fluid in her lungs appeared to be clearing when she went into cardiac arrest and died April 20. La Santa’s family said she had a preexisting heart condition.

Skylar Herbert, a 5-year-old from the Detroit area, died April 16. Her mother, a Detroit police officer, and her father, a firefighter, said Skylar had complained of a headache that wouldn’t relent even with pain medicine. She was initially admitted to a hospital and released but was back six hours later. While in the car, she said her head hurt again, threw up and had a seizure. She later developed meningoencephalitis, which involves the swelling of brain tissue and the brain itself, and was put on a ventilator.

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