We thank Yan Zhang, Wanling Yang and Yu Lung Lau of the University of Hong Kong for assistance in genotyping samples and critical comments on the manuscript. Financial support was provided by Eleven-Fifth Mega-Scientific Project on “prevention and treatment of AIDS, viral hepatitis and other infectious diseases” (2008ZX10003-005,2008ZX10003-012) and National Science Foundation (30872258) of China, and Research Grants Council (778609M, N_HKU752/10) and Research Fund for the Control of Infectious Diseases (10091262) of Hong Kong.