When, where, and how do immunizations fail?

There are two main reasons for failure of immunizations: (1) failure of the vaccine delivery system to provide potent vaccines properly to persons in need; and (2) failure of the immune response, whether due to inadequacies of the vaccine or factors inherent in the host. The first category is by far the most important worldwide. The major factor contributing to failure of the delivery system is failure to vaccinate; in the developing world this is commonly a result of inadequacy of the vaccine supply. Other important factors include barriers to immunizations, improper use of vaccines, vaccine ineffectiveness at the time of use, and factors relating to client attitudes and knowledge. Failure of the immune response may be either primary or secondary (loss of protection after initial effectiveness). The shortcomings of existing vaccines must not deter us from taking maximal advantage of their benefits.