CD4 and COVID-19: It is well-known that naive CD4+ T cells are involved in the induction of cellular responses for new antigens; thus, the significant reduction in these cells found here allows us to putatively suggest that the immunological challenge imposed by the vaccination for COVID-19 in the volunteers of the PE group was able to activate these cells, improving the conditions of B cells to produce antibodies due to their function of the helper immune cells [62,63].