CD4 and susceptibility to pneumonia measurement: CD4+ T lymphopenia in circulation is also a major comorbidity of severe/critical COVID-19 patients who suffer from sepsis.25,26 We also observed that the inverse correlation of CD4+ (Fig. 1e) and CD8+ T cell counts (Supplementary Fig. 1h) and PICC levels (IL-6, IL-1β, IL-8 and IFNγ) in bronchoalveolar lavage fluids of COVID-19 patients hospitalized in 2020 (Supplementary Table 1), whose symptoms transformed from severe to mild pneumonia after symptomatic treatments (Fig. 1f).