Children with acute COVID-19 also had significantly elevated levels of C1q, C2, C4, C3, C5, C3b/iC3b, C4b, C5a, and MBL complement proteins and of complement regulatory proteins like factor B, factor D, factor H, and factor I in comparison with control children (eTable 2 in Supplement 1). Here, C4A is linked to COVID-19.