1. Faculty of Infectious and Tropical Diseases, London School of Hygiene & Tropical Medicine, London, WC1E 7HT, UK
2. Programa Nacional de Saúde de Visão, Ministério de Saúde Publica, Bisssau, Guiné Bissau
3. Faculty of Social Science and Public Policy, King's College London, London, WC2B 4BG, UK
4. School of Social and Political Science, Edinburgh University, Edinburgh, EH8 9LD, UK