Hong Kong Red Cross Blood Transfusion Service, Hong Kong, SAR, China. Public Health Laboratory Centre, Centre for Health Protection, Department of Health, Hong Kong, SAR, China. Department of Pathology, Queen Elizabeth Hospital, Hong Kong, SAR, China. State Key Laboratory for Emerging Infectious Diseases, Carol Yu Centre for Infection, Department of Microbiology, Li Ka Shing Faculty of Medicine, The University of Hong Kong.