Immunization protects community health in two ways. The more obvious is through protecting individuals from communicable diseases. The second is the promotion of herd immunity. If an immunization were 100% effective (an immunized person had a zero probability of becoming infected with the target disease), then immunizing every person in a community would eradicate the target disease in that community. In this situation, personal protection and community protection are the same.

If everyone in the community were immunized save one, that one unimmunized person would also be protected from infection. Assuming that the community is isolated from other, unimmunized communities, the single unimmunized individual is protected because the disease has been eradicated. The problem is that immunizations have risks, so every person wants to be the one unimmunized individual protected by the herd.

Herd immunity is critical to the control of immunizable diseases. It is difficult, however, to predict the precise level of community immunity that is necessary to prevent the spread of a disease. Herd immunity is dependent on the communicability of the disease, the nature of transmission, the effectiveness of the immunization, the duration of period that an infected person may communicate the disease, whether the disease is treatable, whether the disease has a silent period when it is communicable, the general health of the community, and the health of the infected individuals.

Herd immunity is also dependent on the dynamics of a given epidemic. What might be an acceptable level of immunity to prevent spread from random, single cases of the disease could be insufficient to stop a major inoculation such as the arrival of a planeload of infected refugees. Most critically, herd immunity depends on the unimmunized individuals’ being randomly distributed in the community. If unimmunized persons cluster, then it is the percentage of immunization in the cluster that determines the spread of the disease. This is frequently the case with religious groups that refuse immunization. One hundred unimmunized persons in a large city will not be a problem until they all meet at church.

The inherent uncertainty in herd immunity limits its use as an explicit disease control strategy. A community must attempt to immunize every susceptible individual. Herd immunity will then cover the small number of susceptible persons who are inadvertently missed or who are not candidates for immunization. (Herd immunity is the only realistic defense for many immunocompromised individuals.) A community immunization plan must target the individuals who are weak links in a herd immunity system. The largest group is the poor, especially the medically indigent. These individuals are both less likely to be immunized and more susceptible to infection.