CD4 and infection: In individuals with prior infection, specific CD8 T cell levels towards spike of the parental strain or of the Omicron subvariants BA.1, BA.2 or BA.4/5 correlated with IgG titers, and neutralizing activity towards the parental SARS-CoV-2, and in part towards Omicron BA.1 or BA.2, whereas specific CD4 T cells did not show any correlation with humoral immunity (Fig. 4a).