Most children with MIS-C experienced only mild coronavirus symptoms prior to inflammatory illness: study

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Most children who developed the rare but serious multisystem inflammatory syndrome in children (MIS-C) that occurs weeks following a coronavirus infection had no or only mild symptoms of COVID-19 when they were initially infected, according to a new study that’s said to be the largest analysis of such cases in children in the U.S. 

The study, which was led by researchers with the Centers for Disease Control and Prevention (CDC) and published in the medical journal JAMA Pediatrics on Tuesday, looked at nearly 1,800 cases of MIS-C reported to the CDC between March 2020 and mid-January of this year. The study included children up to age 20, though most were younger than 15. 

MIS-C is a condition that often causes different parts of the body to become inflamed, including the heart, lungs, kidneys, brain, skin, eyes, or gastrointestinal organs, according to the CDC, which on its website shows that state-reported cases through March 29 total 3,185 and included 36 deaths. MIS-C may be mistaken for Kawasaki disease, a rare condition that can cause red skin, swelling and heart problems, but the two are not the same. The vast majority of children with MIS-C recover, though some may require hospitalization. 

The study authors found that the majority of the patients studied — about 75% — did not experience symptoms of COVID-19 when they had a coronavirus infection. However, when the patients later developed MIS-C, typically some two to five weeks later, fever was among the most common symptoms, the CDC analysis found. 

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Abdominal pain, vomiting, diarrhea and red skin rash occurred in at least half of the affected kids, while almost one-third had heart inflammation or other cardiac involvement. These symptoms were least common among children up to age 4, who were also less likely to require intensive care than older children.

The CDC in the study said their findings add to existing theories that MIS-C is likely a delayed immune response to a COVID-19 infection. 

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"In this cross-sectional study of a large cohort of patients with MIS-C, 2 peaks that followed COVID-19 peaks by 2 to 5 weeks were identified. The geographic and temporal association of MIS-C with the COVID-19 pandemic suggested that MIS-C resulted from delayed immunologic responses to SARS-CoV-2 infection. The clinical manifestations varied by age and by presence or absence of preceding COVID-19," per the study.

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Treatments for MIS-C often include steroids to reduce inflammation. Though the condition can make some children very ill — one Michigan boy, for example, underwent four amputations of his hands and legs after developing MIS-C — most children who develop it make a full recovery. 

Experts have stressed that coronavirus vaccines are the best way to prevent COVID-19 infections, with COVID-19 vaccine studies in children currently underway. Pfizer and BioNTech, for instance, announced late last month that its Phase 3 trial involving its coronavirus vaccine in adolescents ages 12 to 15 was found to be safe and 100% effective.

"The clinical manifestations of patients with MIS-C varied by age and presence or absence of preceding COVID-19. Development of laboratory markers or diagnostic methods to distinguish MIS-C from severe COVID-19 illness and other hyperinflammatory conditions, such as Kawasaki disease, is critical for early and prompt diagnosis and treatment," the study authors concluded, in part. "As the COVID-19 pandemic spreads, causing a third peak and more sustained transmission across the United States, physicians should maintain a high index of suspicion for MIS-C to promptly diagnose and treat these patients. Practitioners should report patients suspected with MIS-C to local and state health departments."

Fox News' Kayla Rivas and the Associated Press contributed to this report.