Even though not all vaccines are 100% effective, they all offer significantly more protection from disease than not getting vaccinated at all. Think of seat belts – they don’t prevent serious injury in car accidents 100% of the time, but we know that passengers are much better off wearing one because people are better protected during a crash than if they choose not to buckle up. In cases where vaccines don’t fully prevent a person from getting a disease, they do help reduce the severity of the symptoms and the length of time a person is sick.
For some diseases such as smallpox, polio, and diphtheria, the vaccines have reduced the number of cases by 100 percent in the U.S. Since vaccines were developed for measles, mumps, rubella, and Hib, the number of cases have decreased by 99 percent. Varicella (chickenpox) vaccine efficacy ranges from 70 – 100%, but those who experience breakthrough infection have significantly milder illness, generally without fever and with few lesions.
The chart linked below shows the percentage decrease of infection from vaccine-preventable diseases in the U.S.