CD28 and COVID-19: Moreover, a recent study has shown that aging can alter subset clustering of both B and T cells, in particular, hospitalized COVID-19 patients > 70 years of age (n = 10), when compared to hospitalized COVID-19 patients < 60 years of age (n = 7), showed decreased blood levels of both CD8 + naïve T cells (CD8+, CCR7+, CD45RA+, CD45R0-, CD27+, CD28+) and CD4 + memory T cells (CD4+, CCR7+, CD54RA-, CD45R0+, CD27+, CD28+), these were also characterized by a decreased proliferation index.