1. Department of Medical Statistics and Epidemiology, School of Public Health, Sun Yat-sen University, Guangzhou, China
2. Department of Immunization Programme Planning, Guangzhou Center for Disease Control and Prevention, Guangzhou, China
3. Departments of Environmental Health Sciences and Epidemiology and Biostatistics, School of Public Health, University at Albany, State University of New York, Albany, New York, USA
4. Department of Communicable Disease Control and Prevention, Guangzhou Center for Disease Control and Prevention, Guangzhou, China