1. Department of Mathematics; Shanghai Maritime University; Shanghai 201306 China
2. Rohani Lab; University of Michigan; Ann Arbor MI 48104 USA
3. Department of Mathematics and Statistics; York University; Toronto ON M3J1P3 Canada
4. Centre for Disease Modelling; York Institute for Health Research; Toronto ON M3J1P3 Canada
5. Department of Applied Mathematics; Westren University; London ON N6A5B7 Canada