Abstract
Multisystem inflammatory syndrome in children （MIS-C） is a severe multiple organ system inflammation, including circulatory and gastrointestinal organs, which occurs following severe acute respiratory syndrome coronavirus 2 （SARS-CoV-2） infection, with Kawasaki disease （KD）-like symptoms. However, unlike the Delta variant, there have been few reported cases of MIS-C due to the SARS-CoV-2 Omicron variant. Here, we report a case of MIS-C following SARS-CoV-2 Omicron variant （BA1） infection. A 12-month-old Japanese boy who had been exposed to his sister who was infected with SARS-CoV-2 a week earlier presented with a 5-day history of fever, five KD-like symptoms except for extremity symptoms, no redness at the Bacille Calmette–Guérin vaccination site, and diarrhea. Laboratory tests showed slightly elevated serum inflammatory markers and SARS-CoV-2 Omicron variant （BA1） positivity confirmed through a polymerase chain reaction test, which are consistent with the WHO MIS-C criteria. Intravenous immunoglobulin （2g/kg） and oral aspirin （30mg/kg/day） were administered. During hospitalization, shock-like symptoms and echocardiographic abnormalities were not observed. A case of MIS-C associated with the Omicron variant, resembling ours had been reported. This case presented with KD-like symptoms following a mild clinical course without shock-like symptoms and echocardiographic abnormalities. This suggests that the Omicron variant of SARS-CoV-2 infection is less severe than the Delta variant, subsequently reducing MIS-C severity. The patient met all of the MIS-C criteria set by the WHO but failed to meet those set by the Centers for Disease Control and Prevention and pediatric inflammatory multisystem syndrome criteria by the Royal College of Pediatrics and Child Health. This suggests that mild MIS-C can only be diagnosed by following the WHO criteria. In conclusion, MIS-C due to the Omicron variant may be milder than that caused by the Delta variant and may be more difficult to distinguish from KD.
