Serological analysis of severe acute respiratory syndrome coronavirus 2 infection in children with Kawasaki disease.

Multisystem inflammatory syndrome in children (MIS-C) was reported as a severe complication of coronavirus disease 2019; an infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and was suggested to be associated with Kawasaki disease (KD) in terms of severe systemic inflammation and mucocutaneous symptoms. Because severe gastrointestinal symptoms and systemic shock are more frequently observed with MIS-C, patients with mild MIS-C might have been diagnosed with KD. In this study, titers of IgG antibodies against the SARS-CoV-2 S (S-IgG) and N proteins (N-IgG) were measured in 99 serum samples collected from patients with KD treated between January 2020 and December 2021 to evaluate the relationship between KD and SARS-CoV-2 infection. S-IgG were detected in only one patient out of 99 patients. This patient had coronavirus disease 2019 (COVID-19) 10 months before KD onset, and was unlikely MIS-C. According to characters of S-IgG and N-IgG, the patients was unlikely infected with SARS-CoV-2 just before the onset of KD. In addition to this study, the 26th Nationwide Survey and previous studies showed an association between KD and SARS-CoV-2 to be unlikely. In conclusion, SARS-CoV-2 infection was not observed in patients with KD until Delta predominance in Japan by the method of detecting SARS-CoV-2 IgG.