Lymphopenia was commonly found in COVID-19 patients10,11 and is closely correlated with the illness severity.24,25 Laboratory results showed that the counts and percentages of lymphocytes including CD4+ T, CD8+ cytotoxic T,26,27 natural killer (NK),24 and B cells25 were all reduced in COVID-19 patients.28–31 Evidence from single-cell sequencing,32 flow cytometry,33 and nonhuman primate models of COVID-1934 confirmed the involvement of lymphopenia to the maximum extent. Here, CD4 is linked to COVID-19.