Childhood immunization has been called our society's greatest health care achievement. The development and widespread use of vaccines has led to the reduction or eradication of once common childhood diseases.

Unfortunately, vaccines are not 100% effective. Some people do not respond to the protection offered by a vaccine and since the vaccine does not work for these people they remain at risk for disease. Also, the effectiveness of a vaccine depends on the number of people in a community who have received the vaccine.

This is where community immunity, or "herd immunity," comes in. High levels of community immunity are central to protecting people in communities from disease. Vaccinated people are much less likely to spread a germ to other people and act as "protectors" by slowing down or preventing the spread of disease. Thus, even people who have not been vaccinated (or those who do not respond to the vaccine) can be protected through community immunity because vaccinated people around them are not getting sick. Typically, unvaccinated persons are those that cannot be given a vaccine due to other medical conditions, those that are not yet up-to-date with their shots, or those who have chosen not to vaccinate (vaccine exemptors). Note, unvaccinated people are more likely to catch and spread disease and, in one study*, exemptors were 35 times more likely to contract measles than were vaccinated persons.

Community immunity is more effective as the percentage of people vaccinated increases. Research states that approximately 95% of the people in the community must be protected by a vaccine to achieve herd immunity. People who are not immunized increase the chance that they and others will get the disease. Thus, community immunity is thought of as a public good.

While the benefit of herd immunity is great, for some diseases, community immunity offers no protection. For example, tetanus is not contagious. It is contracted when a wound comes in contact with soil contaminated with the tetanus bacterium. To protect you and your family - vaccinate!

For more on this topic, visit the National Network for Immunization Information.

*Salmon DA, Haber M, Gangarosa EJ, Phillips L, Smith NJ, and Chen RT (1999). Health Consequences of Religious and Philosophical Exemptions From Immunization Laws: Individual and Societal Risk of Measles. JAMA 282:47-53