Indeed, heterologous prime-boost immunization regimens per se maybe more beneficial, i.e., ChAdOx1 nCoV-19 and mRNA-1273 or ChAdOx1 nCoV-19 and BioNTech have been shown to augment COVID-19 vaccine efficacy by enhancing spike-specific IgG, nAbs as well as CD4+ and CD8+ T-cells including robust recognition of variants of concern above levels induced by homologous vaccination (67–71). Here, CD4 is linked to COVID-19.