CD4 and COVID-19: However, several studies have reported that the COVID-19 vaccine shows high interpersonal variability in terms of humoral and cellular responses, such as those with respect to anti-SARS-CoV-2 spike immunoglobulin (Ig)G [11,12,13,14,15,16,17], IgA [15,16], IgM [16], neutralizing antibodies [18,19,20], and CD4+ and CD8+ T cells [18,19,21].