Each day we make choices that require weighing the risk to ourselves or the people who depend on us. The risk of a serious adverse event (defined as hospitalization, permanent disability, life-threatening illness, or death) from a vaccine is extremely low but for some people, no amount of risk is acceptable. With that in mind, consider the risk of serious illness or death that comes with the choice not to vaccinate -- a risk that grows as more people decline to vaccinate their children.
For example, doctors in Colorado decided to track vaccination choices following an upswing in cases of pertussis (whooping cough) that caused 140 deaths from 2000 to 2005. They checked the medical records of children who contracted pertussis to see if the parents had refused vaccination for their children. The results showed that children who had not been vaccinated were 23% more likely to get pertussis, a disease that causes uncontrollable coughing lasting for weeks or months, severe vomiting, blue lips, and sometimes death.
Vaccines are not perfect but the point is that those who do not receive the vaccine are the people who are most likely to succumb to a preventable disease should it begin to spread.