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Life-cycle of an Immunization Program

Like a good story, a good immunization program has a beginning, a middle, and, ideally, an end.  The following graphic describes the life-cycle of a typical immunization program.  It illustrates, among other things, why a disease that has been nearly eliminated through vaccination might suddenly begin to reappear. 

For example, it can help explain why there are outbreaks of diseases like measles and pertussis occurring in the United States today.

Graph showing the life-cycle of an immunization program

  1. When there is no vaccine for a disease, the number of people getting the disease is usually high. People worry about the disease and its complications.
  2. After an immunization program for a disease begins, the number of people being vaccinated usually rises quickly.
  3. At the same time, there will be some adverse reactions associated with the vaccine — almost always very few and very mild compared with illness and complications associated with the disease. (Note that this number remains fairly constant because it is a percentage of the number of people being vaccinated.)
  4. As the number of people being vaccinated rises, the number of cases of disease drops. Eventually, the number of people getting the disease may approach, or even fall below, the small number of people having reactions from the vaccine.
  5. At this point, most people may never have experienced the disease. They might start to worry less about the disease and more about possible side-effects from the vaccine. They might start to question whether the vaccine is necessary or safe, and some of them will stop getting immunized.
  6. If enough people stop getting immunized, disease numbers will start to rise again, and there will be disease outbreaks.
  7. People are reminded of how bad the disease can be, and turn back to immunization to avoid it. Vaccination numbers rise once more and disease numbers fall.
  8. Ultimately, if enough people get immunized the disease will disappear altogether. So far this has happened once, with smallpox.
  9. When there is no more disease, the immunization program can be stopped. The numbers of vaccinations and adverse reactions drop to zero.


Image and content adapted from:
Chen RT, Rastogi SC, Mullen JR, Hayes S, Cochi SL, Donlon JA, Wassilak SG. The Vaccine Adverse Event Reporting System (VAERS). Vaccine 1994;12:542-50.

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