1. School of Mathematical and Physical Sciences University of Technology Sydney Ultimo NSW 2007 Australia
2. ARC Centre of Excellence for Mathematical and Statistical Frontiers Parkville Victoria Australia
3. Department of Statistics North Carolina State University Raleigh NC 27695 USA
4. Division of Pulmonary and Critical Care, Department of Medicine Johns Hopkins University Baltimore MD 21218 USA
5. Program in Global Disease Epidemiology and Control, Department of International Health Bloomberg School of Public Health Johns Hopkins University Baltimore MD 21218 USA