Immunization can be given as prophylactic or therapeutic dose. Most vaccines are given prophylactically, i.e., prior to exposure to the pathogen.
However, some vaccines can be administered therapeutically, i.e., post exposure (eg. rabies virus). The effectiveness of this mode of immunization depends on the rate of replication of the pathogen, incubation period and pathogenic mechanism.
In a situation where pathogen has a short incubation period, the pathogenic mechanism is such that only a small amount of pathogenic molecules could be fatal (eg. tetanus and diphtheria) and /or bolus of infection is relatively large, both passive and active post exposure immunizations are essential.
Passive prophylactic immunization is also needed in case of defects in immune system, such as hypo gamma globulinemia.
Pessimistic Side of Immunization
Active immunization may cause fever, malaise and discomfort. Some vaccines may also cause joint pains or arthritis (rubella), convulsions, sometimes fatal pertussis, or neurological disorders (influenza).
Allergy to egg may develop as a consequence of viral vaccines produced in egg (measles, mumps, influenza, yellow fever). Booster shots result in more pronounced inflammatory effects than the primary immunization.
The noticeable and serious side effects documented have been those following the DPT vaccine. Most of these were attributable to the whole pertussis component of the vaccine and have been eliminated since the use of the acellular pertussis preparation.