CD4 and susceptibility to pneumonia measurement: For example, our patients with COVID-19 who had confirmed pneumonia had elevated percentages of activated CD8+ T-lymphocytes, activated CD4+ T-lymphocytes, NK cells, monocytes, and eosinophils but lower percentages of B-lymphocytes and immature B-lymphocytes as well as lower percentages of T-lymphocytes expressing checkpoint molecules PD-1 and CTLA-4 at post-COVID sampling compared to patients without pneumonia.