1. Mahidol-Oxford Tropical Medicine Research Unit, Faculty of Tropical Medicine, Mahidol University , Bangkok 10400 , Thailand
2. Centre for Tropical Medicine and Global Health, Nuffield Department of Medicine, University of Oxford , Oxford OX3 7LG , UK
3. Social and Preventive Medicine Department, Chiang Rai Prachanukroh Hospital , Chiang Rai 57000 , Thailand
4. Nuffield Department of Primary Care Health Sciences, University of Oxford , Oxford OX2 6GG , UK