Although vaccines have very high effectiveness rates, they are not completely effective for 100% of the people who receive them. For example, a full series of measles vaccine will protect 99 of 100 children from measles, and polio vaccine will protect 99 of 100 children from polio.(2) This means that when there is a disease outbreak, the very small number of people for whom the vaccine did not work may still be able to catch the disease. Because almost all of our children are immunized, and only few are not, it can be the case that during an epidemic the majority of cases occur among children who were immunized. However, the fact remains that those who have not received the vaccine are much more likely to catch the disease.
- By way of example, consider an actual measles outbreak in Colorado in December 1994.(5) Out of 625 children exposed to the disease, 17 got measles. Of those 609 who had previously been vaccinated, only 10 (or 1.6%) developed measles. Of the 16 children who were not immunized, 7 (or 44%) developed measles. Thus, the risk for immunized children was less than 2% while the risk for unimmunized children was 44%.