1. Department of Applied Mathematics, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong (SAR), People's Republic of China
2. Department of Biology, McMaster University, Hamilton, Ontario, Canada L8S 4L8
3. M.G. DeGroote Institute for Infectious Disease Research, McMaster University, Hamilton, Ontario, Canada L8S 4L8
4. Division of Mathematics, University of Dundee, Dundee DD1 4HN, UK
5. Department of Mathematics and Statistics, McMaster University, Hamilton, Ontario, Canada L8S 4K1