CD4 and COVID-19: Patients with a systematic oncological treatment (such as chemotherapy or radiotherapy) are not only at a higher risk of more serious COVID-19 infections due to hampered cellular host immune responses due to a gain of pro-inflammatory IL-17-producing T cells and cytotoxic T cells as well as a decline of CD4 T cells and CD8 T cells [50,51], but also a link on a molecular level between cancer and COVID-19 has been proposed.