The reproduction number,R,is the average number of sec-ondary infectious cases produced by one infectious case during a disease outbreak[1].When a population is totally susceptible,R becomes the basic reproduction number,Ro.It is a key parameter regulating the transmission dynamics of a pandemic[2].Ro pro-vides an indication of whether the introduction of disease will result in a localized burnout or signal the beginning of a pandemic that could move through all geographic scales[3].The disease-free equilibrium is globally asymptotically stable and the epidemic eventually disappears if Ro1,then an epidemic will occur and the infection spreads in the population,no matter how small the initial number of infected individuals.When Ro>2,a major outbreak is possible.When Ro>3,the emergence of a pandemic is generally considered to be inevitable[4].