1 Ministry of Health, Singapore, London School of Hygiene & Tropical Medicine, London, UK; Centre for the Mathematical Modelling of infectious Diseases, London School of Hygiene & Tropical Medicine, London, UK; Department of Infectious Disease Epidemiology, London School of Hygiene & Tropical Medicine, London, UK. Electronic address: rachaelpung@hotmail.com.
2 National Public Health Laboratory, National Centre for Infectious Diseases, Singapore.
3 Centre for the Mathematical Modelling of infectious Diseases, London School of Hygiene & Tropical Medicine, London, UK; Department of Infectious Disease Epidemiology, London School of Hygiene & Tropical Medicine, London, UK.
4 Ministry of Health, Singapore, London School of Hygiene & Tropical Medicine, London, UK; Saw Swee Hock School of Public Health, National University of Singapore, Singapore.