1. Department of Infectious Disease Epidemiology, Faculty of Epidemiology and Population Health, London School of Hygiene & Tropical Medicine, Keppel Street, London, WC1E 7HT, England.
2. School of Paediatrics and Child Health, University of Western Australia, Crawley, Australia.
3. Department of Community Health, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana.
4. Kintampo Health Research Centre, Kintampo, Ghana.