After infection, susceptible individuals S enter the exposed class
E before they become infectious individuals I and either recover
(R) or die (D). The average durations of incubation and infectiousness
are given by 1/ and 1/, respectively. f is the case fatality
rate. The transmission rate before the introduction of control interventions
was assumed to be constant, i.e., ˇ(t) =ˇ0. Upon the
implementation of control measures at time , the transmission
rate was assumed to decay exponentially: ˇ(t) =ˇ0e−k(t−) (Lekone
and Finkenstädt, 2006). The basic and net reproduction numbers
are given by R0 =ˇ0S(0)/ and Rt =ˇ(t)S(t)/, respectively.
We assumed the outbreak started with a single infected case
in a large susceptible population (I(0) = 1 and S(0) = 106). As long
as the number of cases is small compared to the total population
size, the exact number of susceptible individuals does not need
to be known to estimate the model parameters. The ODEs were
solved numerically in the R software environment for statistical
computing (Development Core Team, 2014) using the function ode
from the package deSolve.
We