Moreover, we found an 2.5-fold increase of C9 in non-critical patients and a down-regulation of CFH; these findings suggest that, in critical COVID-19 patients, the C9 complement component might be impaired due to SARS-CoV-2 infection, as has already been shown for the hepatitis C virus (Kim et al., 2013). This evidence concerns the gene C9 and COVID-19.