1 Department of Epidemiology and Biostatistics, School of Public Health, Imperial College London, London W2 1PG, UK. Electronic address: f.piel@imperial.ac.uk.
2 Department of Pediatrics, Vanderbilt-Meharry Center of Excellence for Sickle Cell Disease, Vanderbilt University Medical Center, Nashville, TN, USA.
3 Department of Haematology and Blood Transfusion, College of Health Sciences and Centre of Excellence for Sickle Cell Disease Research and Training, University of Abuja, Abuja, Nigeria.