When examining the COVID-19-reactive T-cells in patients and unexposed persons, it has been seen that CD4+ T cells that can recognize the COVID-19 S protein are present in 35% of the unexposed individuals, and at least 40% of the unexposed individuals have CD4+ T cells that are cross-reactive to other COVID-19 proteins than just the S protein [57,58]. Here, CD4 is linked to COVID-19.