CD4 and COVID-19: We found that the patients who had severe COVID-19 had low or absent cellular immune responses of CD4+IL-2+, CD4+IFN-γ+IL-2+, CD8+IFN-γ+IL-2+, and CD8+IFN-γ+IL-2+TNF-α+ after a booster dose (Figure 6), in line with the relevance of polyfunctional T cell responses against viruses [35].