Let’s consider a highly-contagious, fictional disease by the name of Sauderosis. Assume we have a vaccine for Sauderosis which is effective in preventing the disease in 90% of the people who are vaccinated and subsequently exposed to the disease.
Imagine a school with one hundred students who have all been vaccinated against Sauderosis. Since the vaccine is 90% effective, ten of the students in the school will be susceptible to Sauderosis if they are exposed. If there are 20 students in each class, that means two students in each class who are susceptible, so there is not likely to be a widespread problem with the illness in the school, even if someone comes to school sick with the disease.
Now consider that same school if only 50% of the students are immunized against Sauderosis. There are now 55 unprotected students in the school, all 50 who were not vaccinated and five who were (10% of the 50 immunized students). That would mean there are, on average, 11 students in each class who are susceptible so there is a significant chance of a large outbreak if the illness is introduced to the school.
Now imagine your child has an underlying medical condition which makes him more likely to have complications of Sauderosis. If he gets this illness, he is likely to end up in the hospital and may even have life-threatening complications. Which school population would you want him to be in?