CD4 and lymphopenia: Limited by research conditions, our study only demonstrated that severe COVID-19 cases had significant lymphopenia, especially the helper (CD3+CD4+) and cytotoxic T cells (CD3+CD8+), in the early stage of illness onset, consistent with previous research results, and lymphocytes and their subgroups in severe COVID-19 patients require a longer recovery period, compared with severe influenza patients.