S100A9 and COVID-19: The chemokine CXCL10 has also been identified as a plasma biomarker of impaired CD4+ and CD8+ T cell responses in acute COVID-19 (22), and S100A8 and S100A9 alarmins, known to be released by myeloid cells in inflammatory situations, are biomarkers of monocytes and neutrophil subsets alterations (20).