CD8A and COVID-19: In summary, baseline N4R and N3R can be potential biomarkers for assisting in differentiating severe or critically-severe COVID-19, and dynamically monitoring peripheral T-lymphocyte subsets, especially CD3+CD4+ and CD3+CD8+ T-cell numbers during hospitalization 6-9 days after baseline, can help clinicians to evaluate disease progression in COVID-19 patients.