Since the COVID-19 pandemic began last year, more than 3,000 kids have been diagnosed with multisystem inflammatory syndrome in children, a serious condition linked to coronavirus infection that involves organ dysfunction, low blood pressure, fever, and other symptoms, and 36 children have died from it. 

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According to an analysis of hundreds of cases of MIS-C published in JAMA Pediatrics on April 6, most of the patients didn’t report having been ill with their prior SARS-CoV-2 infection before developing the syndrome. Of 1,075 kids with MIS-C for whom the authors had information regarding their bout with COVID-19, just 265 reported a preceding sickness, and low blood pressure, shock, cardiac dysfunction, and admission to the ICU were more prevalent among those who didn’t.

“It means primary-care pediatricians need to have a high index of suspicion for this because Covid is so prevalent in the society and children often have asymptomatic disease as their initial Covid infection,” Jennifer Blumenthal, a pediatric intensivist and infectious disease specialist at Boston Children’s Hospital who was not involved in the study, tells The New York Times.

Doctors first recognized MIS-C’s association with COVID-19 last spring, noting that patients sometimes had mild or asymptomatic cases of the infection beforehand. “They had the infection, they got through it. And then all of a sudden . . . they can have this collapse in their bodies a few weeks later,” Alvaro Moreira, a neonatologist at the University of Texas Health San Antonio, described to The Scientist in September.

In the latest study, physicians from health agencies around the US examined 1,733 reports of MIS-C. Diagnoses peaked in early May, early August, and December, following peaks in COVID-19 cases by two to five weeks. 

The patients commonly experienced fever, abdominal pain, vomiting, rash, diarrhea, and conjunctival hyperemia, when the blood vessels in the eye appear red and dilated. Half of the children had low blood pressure, and nearly one-third developed cardiac complications. Around 58 percent were admitted for intensive care, and 24 patients in the study died.

Another study published last month found that about 10% of children hospitalized with MIS-C presented with acute kidney injury.

It’s not clear why some children develop MIS-C. The authors write in their paper, “Most MIS-C illnesses are believed to result from asymptomatic or mild COVID-19 with hyperinflammation coinciding with peak antibody production several weeks after initial SARS-CoV-2 infection.” A study from last year found that kids with MIS-C are immunologically different from adults with severe COVID-19; in particular, the adults had higher levels of particular cytokines involved in fighting infections.

The risk of developing MIS-C remains very low for kids. According to the American Academy of Pediatrics (AAP) this month, 3.47 million children have tested positive for COVID-19. Sean O’Leary, the vice chairman of the infectious diseases committee for the AAP, tells the Associated Press that most children “respond very well to treatment and the vast majority get completely better.”