A study on COVID-19 vaccines comparing a homologous schedule of an inactivated vaccine (I-I-I) and a heterologous schedule with a recombinant subunit vaccine (I-I-S) reveal that the two strategies differed significantly not only in the induction of neutralizing antibodies but also in the composite pattern of neutralizing antibodies and the population of virus-specific CD4+ T cells produced [23]. This evidence concerns the gene CD4 and COVID-19.