1. School of Mathematics and Statistics, Fuzhou University, Fuzhou 350116, Fujian, China
2. Fujian Provincial Center for Disease Control and Prevention, Fuzhou 350012, China
3. School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China
4. Center for Applied Mathematics of Fujian Province, Fuzhou University, Fuzhou 350116, Fujian, China
5. Key Laboratory of Operations Research and Control of Universities in Fujian, Fuzhou University, Fuzhou 350116, Fujian, China
6. Department of Mathematics and Statistics, University of Strathclyde, Glasgow G1 1XH, UK