CD4 and COVID-19: PCA effectively distinguished the NAb titers of after COVID-19 vaccination using 8 variables, including the CD16+CD56+ NK cell level before the first dose, CD16+CD56+ NK cell level after the second dose, IL-2 level before the first dose, mean corpuscular volume, urea level, and the increment of CD19+ B cells between the second and first doses, the increment of CD4+/CD8+ T cells (also as X48Ratio) between the second and first doses, and the increment of the IL-6 level between the second and first doses in 2-(Figure 2A) or 3-dimensional plots (Figure 2B).