Vaccinations are one of the most important contributions to public health. Vaccines have improved the lives of millions, yet not everyone has been vaccinated. In many cases, every individual does not need to be immune in order to protect most of the population. For instance, as a critical mass of individuals acquire protective immunity either through vaccination or infection they can serve as a buffer for the rest who are not vaccinated. This principle is called herd immunity.
Which of the following is an example of herd immunity?