When a sufficient number of individuals in a population are immune to a disease, as would occur if a large proportion of a popul
ation were vaccinated, herd immunity is achieved. That means that if there is random mixing of individuals within the population, then the pathogen cannot be spread throughout the population.
While this question can appear incomplete, the claim stated is in fact true. There are many people who are deciding that they do not want to vaccinate their children, and this can be bad for herd immunity.
What herd immunity is, is that if 90% of kids in a population are vaccinated (cannot transmit the disease), and they are all spread out, statistically speaking, there is very little chance of that 10% of kids meeting each other.
