If you've got a percentage of the population vaccinated and a percentage of those vaccinated that are still susceptible because the vaccine isn't perfectly effective or effective in all individuals, when you work out the arithmetic you'll see that even with a very effective vaccine, and even if you have the majority of the population vaccinated, the pool of susceptibles will include a number of individuals that have never been vaccinated but will also include quite a few and in some cases greater than 50 percent of the individuals have been previously vaccinated. 