S100A9 and COVID-19: Calprotectin (S100A8/S100A9) is a calcium- and zinc-binding protein mainly secreted by neutrophilic granulocytes or macrophages and has been suggested to be correlated with the severity and prognosis of COVID-19.<h4>Aim</h4>To thoroughly evaluate the diagnostic and prognostic utility of calprotectin in patients with COVID-19 by analyzing relevant studies.<h4>Methods</h4>PubMed, Web of Science, and Cochrane Library were comprehensively searched from inception to August 1, 2023 to retrieve studies about the application of calprotectin in COVID-19.