Live Vaccines

LIVE VACCINES

Live, infective microorganisms, attenuated with respect to their pathogenicity but retaining their ability to infect, can be used to confer protective immunity. Two major advantages stem from the use of live vaccines. First, the immunization mimics the course of a natural infection such that only a single exposure is required to render an individual immune. Secondly, the exposure may be mediated through the natural route of infection (e.g. oral), thereby stimulating an immune response that is appropriate to a particular disease (e.g. secretory antibody as a primary defence against poliomyelitis virus in the gut with oral polio vaccine (OPV); see below). Disadvantages associated with the use of live vaccines are also apparent. Live, attenuated vaccines, administered through the natural route of infection, will replicate in the patient and could be transmitted to others. If attenuation is reduced during the replication process, infections might result (see OPV below). A second, major disadvantage, of live vaccines is that the course of their action, and possible side effects, might be affected by the infection and immunological status of the patient.