Long-lasting immunity against many serious infectious diseases can be elicited through active immunization, the administration of specific antigens (killed or attenuated microorganisms; purified polysaccharides, proteins, or other components; or recombinant antigens produced by genetic engineering) that stimulate the recipient host's production of protective antibodies. Vaccine doses may be given orally, administered as mucosal vaccines, or given by injection using intradermal, subcutaneous, or intramuscular routes. Passive immunization is the process by which protective immunity is obtained through transfer of preformed anti-bodies from an immune host to a nonimmune recipient, either as immunoglobulin or anti-body-specific immunoglobulin.

Protective efficacy resulting from active immunization with a vaccine depends on several factors: the age of the host, with decreased efficacy of certain vaccines observed in the very young and very old; the immune status of the host, with decreased efficacy observed in persons with compromised immune status because of disease or therapy; and the characteristics of the vaccine product itself.

In active immunization, protective levels of specific antibodies usually develop within 2 to 4 weeks on completion of the primary immunization regimen. With the exception of purified polysaccharide vaccines, the antibody response can be recalled and boosted when the immune system is challenged by additional “booster” doses of the vaccine antigen(s) or by exposure to the naturally occurring pathogen.