1. Chinese Center for Disease Control and Prevention, Beijing, People's Republic of China;
2. Center for Disease Control and Prevention of Guangdong Province, Guangzhou, People's Republic of China;
3. Center for Disease Control and Prevention of Guangzhou, Guangzhou, People's Republic of China;
4. Hong Kong Department of Health, Hong Kong Special Administrative Region, People's Republic of China;
5. Queen Mary Hospital, Hong Kong Special Administrative Region, People's Republic of China;
6. Centers For Disease Control and Prevention, Atlanta, Georgia, USA