Government of the United Kingdom - Public Health England, UK Health Security Agency, UK Health Security Agency - Emergency Preparedness, Response and Resilience, Government of the United Kingdom - Public Health England, UK Health Security Agency, Government of the United Kingdom - Public Health England, NHSE/I Children and Young People's Transformation Team, affiliation not provided to SSRN, University College London - Great Ormond Street Institute of Child Health, Cedars Sinai Medical Center - Department of Pediatrics, Government of the United Kingdom - Public Health England, UK Health Security Agency, Public Health England - Immunisation and Countermeasures Division, Royal College of Paediatrics and Child Health, University of Southampton - University Hospital Southampton NHS Foundation Trust, University Hospitals Bristol - University Hospitals Bristol and Weston NHS Foundation Trust, St George's University Hospitals NHS Foundation Trust, Birmingham Women’s and Children’s NHS Foundation Trust, Royal College of Paediatrics and Child Health, NHS Foundation Trust - London North West Healthcare NHS Trust, Alder Hey Children's NHS Foundation Trust, Public Health England, University of Bristol, Public Health England - Immunisation, Hepatitis, and Blood Safety Department, University of Liverpool - Health Protection Research Unit in Emerging and Zoonotic Infections, University of Edinburgh - Department of Child Life and Health, Imperial College Healthcare NHS Trust - Paediatric Infectious Diseases Department, Anuerin Bevan University Health Board, Government of the United Kingdom - Public Health Scotland and Public Health England - Immunisation and Countermeasures Division