In the SARS-CoV infection in 2003, CD4+ responses positively correlated with good outcomes,33 and recent results demonstrated the possible role of T cells in SARS-CoV-2 infection.34 The results from the present study on FNC, which concentrated in the thymus, provided good evidence for the importance of T-cell-mediated anti-SARS-CoV-2 immunity, and further pinpointed that the thymus might be the key organ in the control of COVID-19. Here, CD4 is linked to COVID-19.