Active immunization involves administration of all or part of a microorganism or a modified product of that microorganism (eg, a toxoid, a purified antigen, or an antigen produced by genetic engineering) to evoke an immunologic response that mimics that of natural infection but that usually presents little or no risk to the recipient. Immunization can result in antitoxin, antiadherence, anti-invasive, or neutralizing activity or other types of protective humoral or cellular responses in the recipient. Some immunizing agents provide nearly complete and lifelong protection against disease, some provide partial protection, and some must be readministered at regular intervals to maintain protection. The effectiveness of a vaccine is assessed by evidence of protection against the natural disease. Induction of antibodies commonly is an indirect measure of protection (eg, antitoxin against Clostridium tetani or neutralizing antibody against measles virus), but for some conditions (eg, pertussis), the immunologic response that correlates with protection is poorly understood, and serum antibody concentration does not always predict protection.

Vaccines incorporating an intact infectious agent may contain live-attenuated (weakened), killed (inactivated), or genetically engineered subunits. Vaccines licensed for use in the United States are listed in Table 1.3 . The US Food and Drug Administration (FDA) maintains and updates a Web site listing vaccines licensed for immunization in the United States (www.fda.gov/cber/vaccine/licvacc.htm) . Appendix II shows the years of licensure of vaccines available in the United States, and Appendix III provides the Current Procedural Terminology (CPT) and International Classification of Diseases (ICD-9) codes used for vaccine administration. Many viral vaccines contain live-attenuated virus. Although active infection (with viral replication) ensues after administration of these vaccines, usually little or no adverse host reaction occurs. The vaccines for some viruses (eg, hepatitis A and hepatitis B, human papillomavirus) and most bacteria are inactivated (killed) components, subunit (purified components) preparations, or inactivated toxins or are conjugated chemically to immunobiologically active proteins (eg, tetanus toxoid, nontoxic variant of mutant diphtheria toxin, meningococcal outer membrane protein complex). Viruses and bacteria in inactivated, subunit, and conjugate vaccine preparations are not capable of replicating in the host; therefore, these vaccines must contain a sufficient antigenic mass to stimulate a desired response. Maintenance of long-lasting immunity with inactivated viral or bacterial vaccines may require periodic administration of booster doses. Although inactivated vaccines may not elicit the range of immunologic response provided by live-attenuated agents, efficacy of licensed vaccines is high. For example, an injected inactivated viral vaccine may evoke sufficient serum antibody or cell-mediated immunity but evoke only minimal local antibody in the form of secretory immunoglobulin (Ig) A. Mucosal protection after administration of inactivated vaccines generally is inferior to mucosal immunity induced by live vaccines. Inactivated bacterial polysaccharide conjugate vaccines (eg, Haemophilus and pneumococcal conjugate vaccines) reduce nasopharyngeal colonization through exudated IgG. However, viruses and bacteria in inactivated vaccines cannot replicate in or be excreted by the vaccine recipient as infectious agents and, thereby, cannot adversely affect immunosuppressed vaccinees or contacts of vaccinees.

Table 1-3

Recommendations for dose, vaccine storage and handling (see Vaccine Handling and Storage), route and technique of administration (see Vaccine Administration), and immunization schedules should be followed for predictable, effective immunization (see also disease-specific chapters in Section 3). Adherence to recommended guidelines is critical to the success of immunization practices.