[1] Great Ormond St Hosp Sick Children, Inst Child Hlth, Dept Epidemiol & Publ Hlth, London WC1N 1EH, England
[2] Great Ormond St Hosp Sick Children, Inst Child Hlth, Dept Ophthalmol, London WC1N 1EH, England
[3] Inst Ophthalmol, London WC1N 1EH, England
[4] Inst Ophthalmol, Dept Prevent Ophthalmol, London EC1V 9EI, England
[5] Univ London London Sch Hyg & Trop Med, Dept Infect & Trop Dis, London WC1E 7HT, England