1. Faculty of Population Health Sciences, UCL, UCL Great Ormond Street Institute of Child Health, London, UK
2. Research Department of Infection and Population Health, UCL, London, UK
3. Royal Free London NHS Foundation Trust, London, UK
4. Department of Paediatrics, Hospital Universitario de Getafe, Madrid, Spain
5. Paediatric Infectious Diseases and Immunodeficiencies Unit, Hospital Universitari Vall d’Hebron, Barcelona, Spain