Although all infected children received primary vaccination, a lower prevalence of immunity was observed for all four vaccine-preventable infectious diseases in young HIV-infected patients compared to the uninfected children, which was significantly lower for rubella and hepatitis B. A low seropositivity/seroprotection rate was observed even if patients had, at time of enrollment, a well-controlled HIV infection, as judged by HIV-RNA ≤ 50 copies/mL and CD4 count ≥ 500 cells/mm3. The gene discussed is CD4; the disease is hepatitis B virus infection.