presence of foreign substances, called “antigens,” from, for example, pathogens or vaccines (CDC, 2012d; Siegrist, 2008). In addition to immunoglobulins, other parts of the immune system also contribute to protection, including lymphocytes (specialized white blood cells), antigen-presenting cells (which recognize the foreign elements of the vaccine or the virus or bacterium that is the cause of an infectious disease and which help initiate the steps involved in protection), the spleen, and the skin itself, which serves as a protective barrier against bacteria and viruses.

For a vaccine to be efficacious and reduce the incidence of vaccine-preventable diseases, it must elicit the production of high-quality antibodies against the pathogen responsible for disease. Certain vaccines are able to generate an immunologic memory similar to that generated by natural infection, which often confers lifelong protection, whereas other vaccines may require boosters over time to maintain immunity.

The immune response is largely dependent upon the properties of the antigen used to develop the vaccine and on the route of administration. Live attenuated vaccines contain viruses or bacteria that are weakened versions of the naturally occurring infectious agent, whereas inactivated vaccines contain either antigens that are grown in laboratory culture media and inactivated by the use of heat or chemicals, altered bacterial toxins (toxoids) that when administered do not result in natural disease, or antigens that are produced artificially to mimic the surface properties of the pathogen.

Vaccines containing live, attenuated antigens confer a stronger immune response because the antigen is more similar to that encountered during natural infection; however, in rare cases, the virus may replicate uncontrollably in immunocompromised individuals and lead to a severe or fatal reaction. In an inactivated vaccine, the virus or bacterium is not alive and is not able to cause an infectious disease through unintended replication.

The type of vaccine is one factor that determines where the vaccine appears in the recommended immunization schedule. For example, the measles, mumps, rubella (MMR) vaccine is a live attenuated vaccine that for most recipients confers immunity after just one dose. Children following the recommended immunization schedule receive one dose of MMR at between 12 and 15 months of age and a second dose after age 4 years to ensure immunity. An inactivated vaccine such as diphtheria and tetanus toxoids and acellular pertussis (DTaP) vaccine adsorbed, which contains diphtheria and tetanus toxoids combined with a subunit of the bacterium that causes pertussis, does not confer full immunity until after the second or third dose and requires later booster doses to remain immunologically effective, as the antibody titers that maintain immunity diminish with time.

Adjuvants can provide improved immunity by delaying the absorption of the antigens or by arousing or boosting the immune system response (IOM, 2012; Melvold, 2009). The immunoglobulin M (IgM) isotype is the



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