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Range of acceptable ages for vaccination indicated by colors: First dose ■ Second dose ■ Third dose

Fourth dose

Subsequent doses

Catch-up vaccinations

Fourth dose

Subsequent doses

Catch-up vaccinations single preparation. More of these combination vaccines are being licensed for use in the United States each year.

Current Progress in Immunization

Recent advances in understanding the immune system are enabling researchers to make safer and more effective vaccines. Progress is occurring in several fronts—enhancement of the immune response to vaccines, development of new or improved vaccines against certain diseases, and development of new types of vaccines.

An excellent example of how a better understanding of the immune response can lead to the development of more effective vaccines is the introduction of conjugate vaccines, which enlist T-cell help. Another way in which the immune response can be bolstered is to administer certain cytokines along with vaccines, guiding the immune response. The discovery and characterization of toll-like receptors is giving insights into adjuvants that might be incorporated into vaccines to enhance their effectiveness. Discoveries that lead to enhanced immune responses could facilitate the use of some of the new vaccines currently being investigated. ■toll-like receptors, p. 381

Novel types of vaccines being actively studied include peptide vaccines, edible vaccines, and DNA-based vaccines. Because none of these relies on whole cells, the procedures eliminate the possibility of infection with the immunizing agent; however, some of these vaccines are weakly immunogenic. Peptide vaccines are composed of key antigenic peptides from disease-causing organisms. They are stable to heat and do not contain extraneous materials to cause unwanted reactions or side effects. Edible vaccines are created by transferring genes encoding key antigens from infectious agents into plants. If appropriate plants can be genetically engineered to function as vaccines, they could potentially be grown throughout the world, eliminating difficulties involving transport and storage. DNA-based vaccines are segments of naked DNA from infectious organisms that can be introduced directly into muscle tissue. The host tissue actually expresses the DNA for a short time, producing the microbial antigens encoded by the DNA, which induces an immune response.

There are several serious and widespread diseases for which new or more effective vaccines are currently being sought (table 17.5). Many of these disease-causing agents have been shown to be particularly adept at avoiding the host defenses, complicating the development of long-lasting effective vaccines. In addition to seeking vaccines that protect against infectious diseases, other uses of vaccines are also being studied. Attempts are being made to develop vaccines to control fertility and hormone activity, and to prevent diabetes and cancer, among other conditions. Vaccines are also being used experimentally in the treatment of cancer.

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