[1] UCL Inst Child Hlth, London WC1N 1EH, England
[2] Great Ormond St Hosp Sick Children, Paediat & Neonatal Intens Care Unit, Great Ormond St, London WC1N 3JH, England
[3] Great Ormond St Hosp Sick Children, Childrens Acute Transport Serv, Great Ormond St, London WC1N 3JH, England
[4] Univ Liverpool, Dept Biostat, Liverpool L69 3BX, Merseyside, England
[5] Univ Liverpool, Med Children Clin Trials Unit Univ, Liverpool L69 3BX, Merseyside, England
[6] Barts Hlth NHS Trust, Dept Infect, London, England
[7] St Thomas Hosp, Evelina Childrens Hosp, London, England
[8] Bangor Univ, Ctr Hlth Econ & Med Evaluat, Bangor, Gwynedd, Wales