1 Department of Paediatrics and Child Health, Red Cross War Memorial Children's Hospital, Cape Town 7700, South Africa; South African Medical Research Council Unit on Child and Adolescent Health, University of Cape Town, Cape Town, South Africa. Electronic address: heather.zar@uct.ac.za.
2 Department of Paediatric Respiratory Medicine, National Heart and Lung Institute, Imperial College London, London, UK; Centre for Paediatrics and Child Health, Royal Brompton Hospital, London, UK.