Sickle cell disease (SCD) patients ending with mixed mononuclear chimerism after non-myeloablative HSCT with alemtuzumab/TBI conditioning will probably preserve their immune response to vaccinations administered prior to the transplantation and will therefore not need to be revaccinated. Furthermore, SCD patients after haploidentical HSCT might benefit from adoptive transfer of immunity from their donors. To test the first hypothesis, patients undergoing alemtuzumab/TBI HSCT will be vaccinated with a hepatitis B virus (HBV) vaccine before the transplant. To test the second hypothesis, haploidentical and matched related donors will be vaccinated prior to stem cell donation against HBV. Neither the patient nor the donor may previously have been immunized against HBV in all cohorts. Post-transplantation, the investigators will be able to evaluate whether SCD patients preserve their pre-transplant immune response in the post-transplantation period. Furthermore, the investigators will determine whether donors transfer their immunity to HSCT recipients with SCD disease.