In this observational study, we analyzed the antibody production, the CD4 and CD8 T-cell and the neutralizing antibody response to SARS-CoV-2 Spike protein in 328 subjects (Table 1), including health-care workers (n = 160), elderly people >65 yr (n = 37), and 131 immunocompromised patients with different pathologies including patients in hemodialysis (n = 53), with cancer (n = 30) or rheumatological disease (n = 48) at 2–4 mo (T3) after the second dose of mRNA SARS-CoV-2 vaccination (Spikevax or Comirnaty). This evidence concerns the gene CD4 and cancer.