Vaccines & their Functions
Vaccines are made from either all or portions of the microorganism against which the vaccine protects. These portions of the microorganisms are called antigens and, in most cases, are proteins, glycoproteins, or polysaccharides (Stratton, 2004). There are three traditional types of vaccines. Inactivated vaccines are made from organisms that have been killed in such a way that they still retain most of their original structure. Examples are the inactivated polio vaccine and the influenza vaccine. Live-attenuated vaccines are made of live organisms that have been weakened so that they replicate poorly in the human host and therefore do not cause disease. Examples are measles, rubella, and chickenpox vaccines. Subunit vaccines are made from only some portions of the organism. These often contain toxins that have been altered so that they are no longer dangerous, and are called toxoids, as, for example, the tetanus and the diphtheria vaccines (Levine, 2004). Other subunit vaccines contain recombinant protein antigens, as, for example, the hepatitis B vaccine. A particular class of subunit vaccines is represented by conjugate vaccines, which consist of bacterial polysaccharides linked to protein carriers. An example is the Haemophilus influenzae type B vaccine.
Several types of vaccines are still in the experimental stages and will be part of a new generation of vaccines in the future. These include DNA vaccines , which use the gene encoding a particular protein from an organism, and recombinant vector vaccines , which use an attenuated virus or bacterium to introduce a gene from another microorganism into the host, thus eliciting an immune response against the antigen encoded by that gene. A third type of experimental vaccine is one that does not target microorganisms, but rather is designed to aid in the elimination of cancer cells (Offit, 2003). This type of vaccine, termed dendritic cell vaccine , relies on the host's own dendritic cells, a category of white blood cells responsible for the activation of T lymphocytes, to recognize cancer cells and therefore contribute to the mechanisms necessary for their elimination.
According to Levine (2004), vaccine efficacy is usually improved through the use of adjuvants. These include natural or synthetic compounds used in vaccine formulations that aid in enhancing the immune response to vaccines. Although there are hundreds of compounds being evaluated as vaccine adjuvants, the alum (aluminum hydroxide) adjuvant, first described in 1926, remains the only one used in human vaccines licensed in the United States. Vaccine efficacy is also improved by giving recurrent doses of a vaccine. This practice, called boosting, is done to allow the immune system to remember previous vaccinations by exposing it to the vaccine more than once, and therefore to produce stronger memory immune responses that are effective for an extended period of time (Muraskin, 2005).
The U.S. Food and Drug Administration (FDA) requires extensive research and testing to ensure vaccine safety and efficacy. Before a vaccine can be licensed for general use, preclinical studies must take place, usually in the form of laboratory and animal testing. Researchers test candidate vaccines using cell cultures and animals such as mice, rabbits, guinea pigs, or monkeys. If the vaccine is successful in these preclinical studies, it can go on to be tested in humans (Paoletti, 2006). Human studies involve a series of clinical trials consisting of four phases. Phase I studies enroll up to 20 people and primarily test for safety. Phase II studies involve 50 to several hundred people. These studies continue to test for safety and try to determine the best dosage and gather preliminary data on a vaccine's effectiveness. Phase III studies involve thousands of people and are designed for thorough testing of vaccine efficacy. Finally,...