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.