• Adult patients age ≥ 18 years

• Confirmed haploidentical donor

• Severe SCD phenotype who are at high risk for morbidity and mortality. Severe SCD is defined by at least one of the following:

⁃ i. Clinically significant neurologic event (stroke) or deficit lasting \> 24 hours.

⁃ ii. History of ≥2 acute chest syndromes in a 2-year period preceding enrolment despite optimum treatment, e.g. with hydroxycarbamide (HC).

⁃ iii. History of ≥3 severe pain crises per year in a 2-year period preceding enrolment despite the institution of supportive care measures (e.g. optimum treatment with HC).

⁃ iv. Administration of regular transfusion therapy (=8 packed red blood transfusions per year for 1 year to prevent vaso-occlusive complications).

⁃ v. Patients assessed as requiring transfusion but with red cell allo-antibodies/very rare blood type, rendering it difficult to continue/commence chronic transfusion.

⁃ vi. Patients requiring HC/transfusion for treatment of SCD complications who cannot tolerate either therapy due to significant adverse reactions.

⁃ vii. Established end organ damage relating to SCD, including but not limited to progressive sickle vasculopathy and hepatopathy. End-organ sufficient for entry to this trial shall be ratified at the UK NHP.

⁃ d) Patients must be fit to proceed to Haploidentical SCT as defined below: i. Karnofsky score ≥60 ii. Cardiac function: LVEF ≥45% or shortening fraction ≥25% iii. Lung Function: FEV1, FVC and TLCO ≥50% iv. Renal function: EDTA GFR ≥40 ml/min/1.73m2 v. Hepatic function: ALT \<x3 ULN and bilirubin \<x2 the upper limit of normal, those with hyperbilirubinemia due to sickle related haemolysis will not be excluded. No radiological evidence of cirrhosis.

⁃ e) Written informed consent.