Vaccines induce protection against infections by stimulating the development of long-lived effector cells and memory cells. Most vaccines in routine use today work by inducing humoral immunity, and attempts to stimulate cell-mediated immune responses by vaccination are ongoing. The success of active immunization in eradicating infectious disease is dependent on numerous factors. Vaccines are likely to be must effective against infectious that are limited to human hosts and are caused by poorly infectious agents whose antigens are relatively invariant.
On the other hand, antigenic variation, the existence of animal or environmental reservoirs of infection, and high infectivity of the microbes make it less likely that vaccination alone will eradicate a particular infectious disease.