1. Department of Statistics, The Pennsylvania State University , Shortlidge Rd , State College, PA 16802, USA
2. Research Program, East-West Center, 1601 East-West Road , Honolulu, HI 96848, USA
3. Center for Communicable Disease Dynamics, Department of Epidemiology, Harvard T.H. Chan School of Public Health , 677 Huntington Ave , Boston, MA 02115, USA
4. MRC Centre for Global Infectious Disease Analysis, School of Public Health, Imperial College London , Norfolk Place, London W2 1PG, UK