Densitometric analysis confirmed that the median level of HMGB1 in the serum was significantly different in the three groups (MIS-C: 1,151.38 densitometric units (DU) vs. post COVID-19: 545.90 DU and vs. control: 234.04 DU, p-value < 0.0001 (using the Kruskal–Wallis test). Here, HMGB1 is linked to COVID-19.