Overview.
The dead or weakened germs in vaccines help your child’s immune system to make two important tools: antibodies and immune memory. Together, these tools will help your child recognize and fight off the germs if exposed to them in the future. Most children are fully protected after they are vaccinated. This means that they will never get serious vaccine-preventable diseases.

Children With a Health Problem That Affects Their Immune System.
In rare cases, children who are vaccinated can still get the disease because they only get partial protection from the vaccine. This is more common in children with a health problem that affects their immune system. They may develop mild symptoms if they are exposed to a disease, but will not suffer serious complications.
It’s just like… seatbelts are not 100% effective at protecting you while driving, but they significantly reduce your risk of being injured.