CD4 and infection: Our findings are also in accordance with recent reports, where pre-existing cellular immunity, either by natural infection or immunization, is correlated with recall responses, as Goel et al. reported that Tfh cells induced by a first BNT162b2 dose correlated with antibodies achieved up to 6 months after the second dose [26], while Casado et al. reported that pre-existing CD4 deriving from previous infection or cross-reaction with other coronaviruses correlated positively with specific IgG-antibody titers after the first and second vaccine doses [32].