1. The School of Medicine, Swansea University and Swansea University, Singleton Park, Swansea SA2 8PP, Wales;
2. The School of Health Science, Swansea University, Singleton Park, Swansea SA2 8PP, Wales;
3. Obsidian Research Limited, Port Talbot, West Glamorgan, SA12 7BZ, Wales;
4. London School of Hygiene and Tropical Medicine, London WC1E 7HT, UK