1 MRC Clinical Trials Unit at University College London, Institute of Clinical Trials & Methodology, London WC1V 6LJ, UK; Department of Paediatric Infectious Diseases, Great Ormond Street Hospital for Children NHS Foundation Trust, London, UK. Electronic address: a.turkova@ucl.ac.uk.
2 MRC Clinical Trials Unit at University College London, Institute of Clinical Trials & Methodology, London WC1V 6LJ, UK; Department of Paediatric Infectious Diseases, Great Ormond Street Hospital for Children NHS Foundation Trust, London, UK; University College London Great Ormond Street Institute of Child Health, London, UK.