1 Dalla Lana School of Public Health, University of Toronto, Toronto, M5T 3M7, Canada.
2 Department of Infectious Diseases, Shanghai Key Laboratory of Infectious Diseases and Biosafety Emergency Response, National Medical Center for Infectious Diseases, Huashan Hospital, Fudan University, Shanghai, China; Shanghai Sci-Tech Inno Center for Infection & Immunity, Shanghai, China. Electronic address: zhangwenhong@fudan.edu.cn.