Immunisation and Countermeasures Division, Public Health England, Government of the United Kingdom - Public Health England, Government of the United Kingdom - Public Health England, Joint Modelling Team, United Kingdom Health Security Agency, Public Health England - Immunisation and Countermeasures Division, Immunisation and Countermeasures Division, Public Health England, BPSU, Royal College of Paediatrics, BPSU, Royal College of Paediatrics, Paediatric Intensive Care Unit, Bristol Royal Hospital for Children, Department of congenital cardiology, University Hospital Southampton NHS Foundation Trust, BMBS, Alder Hey Children’s NHS Foundation Trust, Liverpool, Government of the United Kingdom - Birmingham Women’s and Children’s NHS Foundation Trust, Imperial College Healthcare NHS Trust - Paediatric Infectious Diseases Department, Paediatric Intensive Care Unit, St George’s Hospital, Public Health Scotland - Clinical and Public Health Intelligence Team, Public Health Wales, University of Edinburgh - Department of Child Life and Health, University of Liverpool - NIHR Health Protection Research Unit for Emerging and Zoonotic Infection, Public Health England - Immunisation, Hepatitis, and Blood Safety Department, Faculty of Medicine and Institute for Life Sciences, University of Southampton, University Hospitals Bristol NHS Foundation Trust, Government of the United Kingdom - Public Health England and Government of the United Kingdom - Immunisation and Countermeasures Division