**8.2. Alternatives to R0**

R0 is a highly pathogen-centered parameter and recently, a host-centered parameter has been evolved, the basic depression ratio, D0. An alternative parameter for heterogeneous population has been developed, type reproduction number, T which indicates the efficacy of the control measure against a particular subtype of host population, from which if infection is eradicated, the disease will not sustain at all [21].

In case of a population which is susceptible after acquiring immunity from a previous epidemic or due to vaccination, instead of R0, Reff or effective value of R0 is used. Reff is a time-dependent quantity that accounts for the population's reduced susceptibility. If Reff is greater than one, the number of infected individuals grows and decreases if Reff is less than one. Therefore, the critical proportion of susceptible is given by Reff equal to one [11]. The parameter can be determined by fitting deterministic epidemiological model employing a generalized least squares estimation scheme [23].

In case of seasonally driven epidemics as with different childhood microparasitic infections, it is necessary to determine the number of susceptible left after a major epidemic (S0). If S0 goes above a critical threshold value, epidemic outbreak may recur in the next year or there will be a skip i.e., a year when epidemic fails to initiate [14].
