MMP9 and COVID-19: There are limitations in this study: (a) we do not know the original severity of COVID-19 in the patients studied, (b) the results were compared to healthy controls and not recovered long COVID patients, and (c) the source of MMP-9 in the serum of long COVID patients is also not exactly known since SARS-CoV-2 could also stimulate the release of MMP-9 from cultured macrophages [63] and several cell types in the brain [64].