1. Department of Infectious Disease Epidemiology, London School of Hygiene & Tropical Medicine , London, UK
2. Maternal, Adolescent, Reproductive & Child Health Centre, London School of Hygiene & Tropical Medicine , London, UK
3. Department of Clinical Epidemiology, Aarhus University , Aarhus N, Denmark
4. Department of Comparative Biomedical Sciences, School of Veterinary Medicine, University of Surrey , Guildford, UK