CXCL9 and COVID-19: However, further studies are required to investigate the impact of monocytes on the cellular immune response in the context of immunization via COVID-19 vaccination as monocytes exposed to inactivated SARS-CoV-2 were found to secrete higher levels of IL-6, TNF-α, CXCL10, CXCL9, and CXCL11 upon restimulation [25].