The effective reproduction number R(t) measures the number of secondary cases generated by an infectious case once an epidemic is underway. In the absence of control measures, R(t) = R 0 S(t)/N, where S(t)/N is the proportion of the population susceptible [40, 41] . The estimation of reproductive numbers is typically an indirect process because some of the parameters on which these numbers depend are difficult or impossible to quantify directly. The effective reproductive number satisfies R(t) ≤ R 0 , with equality only when the entire population is susceptible [40] .