aUniversity of North Carolina Project-China, Guangzhou, Guangdong, China

bSaw Swee Hock School of Public Health, National University of Singapore and National University Health System, Singapore, Singapore

cDepartment of Medicine, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, USA

dClinical Research Department, London School of Hygiene and Tropical Medicine, London, UK

Correspondence to Dr Rayner Kay Jin Tan, University of North Carolina Project-China, 2 Lujing Road, Guangzhou, Guangdong 510085, China. Tel: +65 91878576; e-mail: [email protected]