“…Over the past few decades, a large number of simple compartmental mathematical models of the general form SVI or SVIR (where S, V, I and R denote the populations of susceptible, vaccinated, infectious and recovered individuals) have been used in the literature to assess the impact or potential impact of imperfect vaccines for combatting the spread of some human diseases (see, for instance, [1,3,4,6,16,17,18,20,24,25,26,35,37] and the references therein). While in some of these studies (e.g., [3,6,16,17,37]) the vaccine is only given to people newly recruited into the population, such as newborns (cohort vaccination), in many others (e.g., in [1,20,26,35]), a proportion of susceptible individuals is continuously vaccinated.…”