Types of Vaccines

12 Key excerpts on "Types of Vaccines"

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  Vaccine Supply and Innovation

  • (Author)
  • 0(Publication Date)
  • The National Academies Press

    (Publisher)

  2 Vaccines: Past, Present, and Future TYPES OF IMMUNIZATION Immunization involves the induction (or administration) of antibodies and other natural defense mechanisms to protect against specific pathogens. There are two types of immunization, active and passive. Active immunization is the major focus of this report. It involves the administration of a modified pathogenic agent, or a component of a pathogen, to stimulate the recipient's immune mechanisms to produce long-lasting protection without causing the clinical manifestations or other consequences of disease. Three major types of preparations are employed to produce active immunity. The first consists of vaccines made from whole, inactivated (killed) pathogens or components of a pathogen. 1 Examples of whole, inactivated vaccines include currently licensed pertussis vaccines, influenza vaccines, and the Salk poliovirus vaccine. The pneumococcal, meningococcal, and hepatitis B vaccines are among those that contain the immunity-producing fraction of the pathogen. Toxoids are the second type of active immunogen. The diphtheria and tetanus vaccines are good examples. Toxoids are toxins that have been treated by physical or chemical means until they no longer produce clinical disease, but retain the capacity to induce immunity. Attenuated infectious vaccines are the third type. Virus vaccines in this group are derived from the offending organism after it has undergone repeated passages in the laboratory in culture; it remains infectious for man but loses the ability to induce clinical disease. Examples include the oral (Sabin) poliovirus vaccine, and the measles, mumps, rubella, and yellow fever vaccines. Other examples of this type of vaccine contain live organisms or agents that are related to but different from the species that causes the disease.

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  New Vaccine Technologies

  • Ronald W. Ellis(Author)
  • 2001(Publication Date)
  • CRC Press

    (Publisher)

  A recent series of live attenuated recombinant vaccines against cholera, typhoid, and shigellosis are at various stages o f human clinical trials. The appli­ cation of recombinant DNA technology to these vaccines will be emphasized throughout this chapter. Types of Vaccines Three types of commercialized vaccines exist: antigen subunits, inactivated organisms, and live attenuated organisms. Vaccines composed of subunits (i.e., purified inactivated proteins/ peptides, carbohydrate capsules, or cross-linked conjugates o f these) are generally considered safest because they are used in small quantities, are chemically well defined, and do not repli­ cate and thus cannot spread to the environment and other non-vaccinated or immuno­ compromised individuals. However, subunit vaccines are often expensive to manufacture and have a limited ability to induce immune responses, requiring adjuvants and multiple doses. Inactivated organisms (i.e., whole bacteria/viruses killed by heat or chemical treatment), although considered very safe, suffer from other drawbacks, such as ill-defined molecular characteristics and poor immunogenicity (i.e., large quantities of vaccine are required to elicit protective immune responses). Inactivated vaccines are relatively easy to manufacture and can possess inherent immunomodulatory activity (e.g., the whole cell pertussis component of DTP), although the New Vaccine Technologies , edited by Ronald W. Ellis. ©2001 Eurekah.com. 152 New Vaccine Technologies spectrum of immune response induced is usually limited to the humoral arm of the immune system .2 In contrast, live attenuated vaccines (i.e., living viruses/bacteria that carry mutations ren­ dering them avirulent or significantly reduced in virulence) offer significant advantages in terms of manufacture and immunogenicity.

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  Applied Pharmacology for Veterinary Technicians - E-Book

  Applied Pharmacology for Veterinary Technicians - E-Book

  • Lisa Martini-Johnson(Author)
  • 2020(Publication Date)
  • Saunders

    (Publisher)

  17: Immunologic Drugs

  1. Principles of Vaccination,
  2. Common Vaccine Types That Produce Active Immunity,
     1. Inactivated (Killed),
     2. Live,
     3. Modified Live (Attenuated),
     4. Recombinant,
     5. Toxoid,
  3. Common Vaccine Types That Produce Passive Immunity,
     1. Antitoxin,
     2. Antiserum,
  4. Other Types of Vaccines,
     1. Autogenous Vaccine,
     2. Mixed Vaccine,
  5. Vaccine Storage, Handling, and Reconstitution,
  6. Administration of Vaccines,
  7. Vaccine Failure,
  8. Adverse Vaccination Responses,
  9. Core Versus Noncore Vaccines,
  10. Vaccinations for Preventive Health Programs,
      1. Canine,
      2. Feline,
      3. Equine,
      4. Bovine,
      5. Sheep and Goats,
  11. Immunotherapeutic Drugs,
      1. Immunostimulants,
         1. Complex Carbohydrates,
         2. Immunomodulatory Bacterins,

  Objectives

  After studying this chapter, you should be able to

  1. Explain the principles associated with vaccination and describe the recommended locations for various feline vaccinations. 2. Describe the differences between infectious and noninfectious vaccines. 3. Discuss the advantages and disadvantages of the many different Types of Vaccines. 4. Discuss the storage, handling, and reconstitution of vaccines. 5. Describe the different routes of administration of vaccines. 6. Discuss vaccine failure and adverse vaccination responses that may occur. 7. List core and noncore vaccines used in various species, as well as common diseases that have available vaccines. 8. List and discuss drugs used in immunotherapy.

  Key Terms Active immunity Adjuvant Anaphylaxis Antibody Antigen Avirulent Bacterin Core vaccines Monovalent Noncore vaccines Passive immunity Polyvalent Preservative Recombinant DNA technology Toxoid Virulence

  Principles of Vaccination

  Keeping animals healthy through the proper use of immunization programs is an important aspect of veterinary medicine. Vaccine protocols (guidelines) recommended by the American Animal Hospital Association (AAHA), American Veterinary Medical Association (AVMA), American Association of Feline Practitioners (AAFP), and American Association of Equine Practitioners (AAEP) should be observed annually. Vaccination guidelines are a source of evidence-based recommendations and expert opinion provided by the AAHA Canine Vaccination Guidelines Task Force (AAHA, 2017 ) and AAFP feline vaccination guidelines. Veterinary technicians must have knowledge concerning vaccine types and the diseases against which animals are vaccinated. Clients ask many questions regarding this area of their pet’s care. As animals enter into their geriatric years, regular laboratory profiles should be done to determine the health of major organ systems. Preventive health care guidelines include a complete history, comprehensive physical examination, assessment, diagnostic plan, therapeutic plan, prevention plan, and a follow-up plan on the animal (JAAHA, 2011

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  Recent Trends In Livestock Innovative Technologies

  • Muhammad Hamid, Hafiz Ishfaq Ahmad(Authors)
  • 2009(Publication Date)
  • Bentham Science Publishers

    (Publisher)

  E. coli O157:H7 and Salmonella in food animals. These vaccines reduce shedding of pathogens from animals. Vaccines have also strengthened the bond between human and companion animals. They can live together without fear of zoonotic diseases. There is much development in the production of new veterinary vaccines, but there is a need to sit together for new emerging diseases. There should also be work on reducing vaccine costs for animals and providing vaccines to developing countries.

  14.2. Types of Vaccines

  Safety concerns of the world led to the development of different types of live attenuated vaccines. Each new type is being developed to boost immunity with fewer side effects. Roughly 4 general categories of vaccines have been made.

  14.2.1. Live Attenuated Vaccine

  Live attenuated vaccine is usually made for viruses and contains organisms that have been weakened to the level that cannot cause disease. There are several methods to develop a live attenuated vaccine but the most common is the growth of the virus several times on the unnatural host cell line or tissue culture. There are several examples for the development of this type of vaccine, including oral Polio vaccine, Influenza, and Rotavirus vaccine [2 ].

  14.2.2. Inactivated Vaccine

  In this method, vaccines are produced through inactivation or killing of the organism through heat or chemicals so that immunogenicity remains intact. The antigenic epitope has to be intact during the inactivation process. Flu, rabies and hepatitis vaccines are produced with this method.

  14.2.3. Recombinant/Subunit Vaccine

  These vaccines only use some parts of the organism, which produces a strong immune response in the body. Bacteria or virus’s structural parts of protein can be used. Protein antigens can also be produced in some other non-pathogenic organisms through genetic engineering of the genome nucleotides. Vaccines for Hepatitis B and whooping cough are examples of this type of vaccine. They may need booster doses for proper immunization.

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  Development and Formulation of Veterinary Dosage Forms

  • Gregory E. Hardee, J. Desmond Baggo, Gregory E Hardee, J D Baggot, Gregory Hardee, J Baggot, Gregory E Hardee, J D Baggot(Authors)
  • 2021(Publication Date)
  • CRC Press

    (Publisher)

  Safety is also a concern in any formulation so all virus vaccines, in their final formulation, are tested at two to 10 times the recommended dose in the most susceptible animal known. If any overt clinical signs are noted or adverse reactions occur the vaccine is considered unsafe and is not offered for sale.

  IV. Bacterial Vaccines

  A. Types

  Vaccines for the prevention of disease caused by bacterial infectious agents may take one of five forms: inactivated toxin (toxoid), chemically inactivated whole organisms (bacterin), attenuated live organism (modified live), recombinant produced and/or purified cellular proteins (subunit), or live recombinant organisms. Of these, the toxoid and chemically inactivated whole organisms make up the majority of vaccines licensed for use in animals.

  Toxoids are prepared from bacteria-produced toxins, which are inactivated and used to prevent or control diseases caused by these toxins. Toxoids are used to control infections caused by members of the following genera; Pasteurella (pneumonia in cattle, pigs, and sheep); Staphylococcus (mastitis); Bordetella bronchiseptica (atrophic rhinitis in pigs); and Clostridia sp., which are responsible for causing such diseases as tetanus in horses; enterotoxemia of lambs; malignant edema of horses, cattle, sheep, and swine; black disease in sheep and sudden death in cattle and pigs. Tenanus toxoid with adjuvant is a highly potent antigen capable of inducing immunity against the tetanospasm. However, at least two doses, 4 to 6 weeks apart, are required to induce solid immunity in the horse. This immunity must be reinforced at yearly intervals to ensure that protective immunity is maintained. The use of toxoids to control disease in companion animals (dogs and cats) is relatively rare.

  Inactivated whole bacterial cell suspensions are probably the most common type of vaccine used in domestic animals. They are generally administered once or twice annually. These vaccines have historically been extremely effective and safe to use in young or pregnant animals. Vaccines prepared in this manner proved highly effective in protecting against respective diseases. Such was the case with early Leptospira vaccines that were prepared from one serovar and provided protection against only that offending serovar. These vaccines provide only short-term immunity with highly specific efficacy. However, whole-cell vaccines prepared from gram-negative organisms (i.e., E. coli and Salmonella) occasionally caused abortion, disseminated intravascular coagulation, or fever in some animals due to the presence of endotoxin. Using whole-cell inactivated vaccines tended to induce primarily circulating antibody, failing to provide immunity against facultative intracellular or obligate intracellular pathogens, which required the induction of cell-mediated immunity (17 ). In addition, these vaccines often failed, particularly with the gram-negative enteric pathogens— e.g., E. coli—and those bacteria responsible for causing pinkeye in cattle (Moraxella bovis

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  Parasitology

  An Integrated Approach

  • Alan Gunn, Sarah J. Pitt(Authors)
  • 2022(Publication Date)
  • Wiley

    (Publisher)

  Table 15.1 ), and their composition is typically more complex than that of drugs. In addition to the antigen(s) stimulating the immune response, vaccines also often contain stabilisers, adjuvants, preservatives, and by‐products from the production process. This complexity means that vaccines are difficult to manufacture although commercial companies consider this beneficial because it limits the ability of a competitor to manufacture a generic version.

  Table 15.1

  Types of vaccine.

  Type of vaccine	Example
  Live attenuated	MMR (measles, mumps, rubella), yellow fever, smallpox
  Inactivated	Rabies, Hepatitis A
  Subunit/recombinant vaccines	PPV (adult pneumococcal vaccine)/HPV (Human papillomavirus vaccine)
  Polysaccharide conjugate vaccines	Haemophilus influenza type b (Hib)
  Toxoid vaccines	DPT: diphtheria, pertussis (whooping cough), tetanus
  Virus‐like particles	None in commercial production
  Peptide/polypeptide vaccines	None in commercial production
  DNA/ RNA vaccines	Pfizer‐BioNTech COVID‐19 vaccine

  After identifying a potential target antigen that is safe and induces protective immunity, a crucial subsequent step is to manufacture it in bulk quantities and then process it for delivery as a commercial vaccine. During these steps, conformational changes to the antigen can occur. For an antigen to induce a protective effect, it is essential for the antigen in the vaccine to retain the same conformation as the intended target antigen on the pathogen. If it does not, the vaccine will generate antibodies that do not attach to the pathogen.

  15.6.1 Live Attenuated Vaccines

  These vaccines utilise pathogens rendered harmless or less virulent (i.e., ‘attenuated’) by passaging them through a foreign host, exposing them to chemicals or radiation, or selectively knocking out genes of pathogens reared in culture. Because these vaccines closely resemble the target pathogen and replicate within the vaccine recipient, it means that the recipient’s immune system must respond to a variety of pathogen life cycle stages. This is an important consideration for antiparasitic vaccines because many parasites undergo a succession of developmental stages in their host. Consequently, live vaccines elicit an immune response like that of the pathogen but without its associated pathology. Live vaccines are therefore more likely to induce a T‐cell‐mediated immune response than killed vaccines, and this is important for combating intracellular parasites.

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  Vaccines For Dummies

  • Megan Coffee, Sharon Perkins(Authors)
  • 2021(Publication Date)
  • For Dummies

    (Publisher)

  Vaccines fall into two broad categories: whole-pathogen vaccines and subunit vaccines. They’re further subdivided into several other categories, including live vaccines, inactivated vaccines, toxoid vaccines, nucleic acid vaccines, and viral vector vaccines. This may seem unnecessarily complicated — can’t science come up with just one type of vaccine that covers a multitude of diseases? The answer is, no they can’t, in most cases, although there are some vaccines that can be combined into one injection.

  In this section, we look at the Types of Vaccines available today and the reason why certain vaccines are made the way they are. Most importantly, we explain how specific vaccines may affect you.

  Whole-pathogen vaccines

  A whole-pathogen vaccine is made from — no surprise here — the whole germ that makes you ill. To continue the “Wanted” poster analogy we start earlier in this chapter, whole-pathogen vaccines are like a “Wanted” poster that shows the whole person. In the same way, whole-pathogen vaccines show your immune system a weakened (live) or dead (inactivated) version of the pathogen. In some cases, a vaccine provides a picture of a very similar pathogen, the way a “Wanted” poster may show a sketch rather than an actual person.

  Scientists aren’t resting on their laurels when it comes to creating vaccines. Vaccines of the future may be able to attack cancer cells before they spread, or to prevent complications from diseases such as HIV from occurring.

  Looking at live vaccines

  Live vaccines are made from weakened or similar versions of the germs that cause disease. These bacterial or viral strains are weakened in the laboratory and cause usually no, or else few, symptoms when they’re given to you.

  After just one or two doses, these vaccines may protect you for years or even a lifetime. They create a long-lasting immune response much like a natural infection would, just minus the worrisome disease. These are some of science’s oldest vaccines. They are tried-and-true, and in some cases, having been used for many decades.

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  Immunology

  Mucosal and Body Surface Defences

  • Andrew E. Williams(Author)
  • 2011(Publication Date)
  • Wiley

    (Publisher)

  The current influenza virus vaccine, administered to high-risk populations in order to prevent epidemics of the flu, is based on inactivated virus. The influenza vaccine is trivalent and consists of three different virus strains, which are chemically inactivated. The influenza A virus strain(s) (highly pathogenic) likely to circulate in the following season are predicted and incorporated into the vaccine together with an influenza B virus strain (mildly pathogenic). Influenza viruses are identified by the type of haemagglutinin and neuraminidase (H and N) proteins they possess. For example, the current vaccine contains H1N1 and H3N2 virus haplotypes plus a strain of influenza B virus (such as the Beijing or Hong Kong strain). However, the inactivated influenza vaccine is not always protective, due to changes in the influenza virus strain as a result of antigenic drift (single mutations in the H and N genes) or antigenic shift (re-assortments of whole gene segments, e.g H3N2 to H5N2). The vaccine is then rendered non-protective as the immune system has never encountered the new virus strain and so the threat of an influenza epidemic or even a pandemic remains. Recent concerns have arisen due to geographically isolated human outbreaks of H5N1 virus, a haplotype thought only to be a problem in birds. Efforts are now being made to produce a H5N1 virus vaccine, although this virus is toxic to chicken embryos, the preferred medium for vaccine virus growth.

  An example of an inactivated bacterial vaccine is the formaldehyde inactivated whole-cell pertussis vaccine (WCPV) administered in childhood for protection against whooping cough. This was first administered in 1914 in the USA and consists of inactivated Bordetella pertussis absorbed onto aluminium salt as an adjuvant. WCPV is usually given in combination with diptheria toxoid and tetanus toxoid (DTwP). A toxoid is the chemically inactivated form of the toxin that causes disease. Although the DTwP vaccine was responsible for a massive decline in the incidence of whooping cough, diptheria and tetanus, some individuals had adverse reactions to the vaccine. The preferred trivalent diptheria, tetanus, pertussis (DTP) vaccine contains an acellular pertussis vaccine (DTaP) containing one or more inactivated components B. pertussis , including the toxin and filamentous haemagglutinin antigen. However, the DTaP vaccine is less efficacious than the DTwP vaccine and requires repeated immunizations.

  17.15 Polysaccharide Vaccines

  Polysaccharide vaccines are also known as conjugate vaccines and consist of bacterial polysaccharides conjugated to proteins. They have been very successful in preventing diseases caused by certain bacteria that possess a polysaccharide capsule, such as Neisseria meningititis (meningitis), Haemophilus influenzae type b (meningitis) and Streptococcus pneumoniae

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  Viruses

  Biology, Applications, and Control

  • David Harper(Author)
  • 2011(Publication Date)
  • Garland Science

    (Publisher)

  In considering the future of human vaccination, it is useful to look at veterinary vaccines. Here, some of the more advanced technologies are already in use. Two DNA vaccines and nine recombinant viral vectors are approved for use in North America, alongside the traditional live and inactivated vaccines. While use in animals does not necessarily lead on to use in man, these must be seen as promising signs. However, when considering the many scientifically intriguing areas of vaccine development, it must be borne in mind that the relatively crude attenuated live vaccines from the last century saved many lives, eliminated one global killer, and are closing on another. In the veterinary field it was a traditional vaccine that now seems to have eliminated rinderpest.

  However vaccines are produced, they remain one of the great success stories of modern health care.

  Key Concepts

  • Vaccination, the use of a weakened form of a disease agent to protect against natural infection, has a 2000-year history. The modern era of vaccination began in 1796 with the work of Edward Jenner.
  • There are four basic types of vaccine in current use: live attenuated (a weakened form of the original virus), inactivated (“killed” virus), subunit (proteins purified from the original virus), and cloned subunit (viral proteins expressed in another system, often yeast cells).
  • Many advanced biotechnological approaches are under development, but as yet the traditional vaccine formulations remain the most widely used.
  • HIV is now (and is likely to remain as) the greatest challenge for viral vaccine development, not least since the type of immunity needed to protect against infection remains undefined.
  • Vaccine efficacy may be potentiated by adjuvants, coadministered chemicals that enhance the immune response. Despite much promise, only a very few are licensed.
  • New routes and methods for the administration of vaccines have the potential to shape the immune response, though understanding of the basis of effective immunity is needed. The sheer complexity of the immune system makes this a challenging area for research.
  • DNA vaccines, naked DNA molecules expressing antigens of choice, promise a rapid and simple vaccine development pathway, but have yet to deliver for humans.
  • The use of therapeutic vaccination (the use of vaccines to control existing disease rather than to prevent new disease) is blurring the lines between vaccines and antiviral drugs, as is the use of immunomodulators and antibody preparations as antiviral agents.
  • Vaccination is the only human intervention to eradicate a major disease, smallpox. Two more virus-induced diseases, polio and measles, are targeted for eradication.

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  Molecular and Cellular Therapeutics

  • David Whitehouse, Ralph Rapley(Authors)
  • 2012(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  There are several different types of DCs of which plasmacytoid DCs and myeloid/monocyte-derived DCs are the most important in T cell responses. DCs are critical to inducing activation of naive TH and CTLs on first exposure to a pathogen. Thus a vaccine must be taken up by DCs to initiate the primary T cell response. One other key feature of DCs is cross-presentation (cross-priming), where DCs present peptides from an exogenous source on both MHC class I and class II molecules. This may be achieved by different mechanisms involving uptake of infected cells, intracellular routing of exogenous protein from the endocytic pathway to the cytosolic pathway or transfer of MHC class I molecules expressing peptides from infected cells to the DCs. Cross-presentation of proteins encoded on plasmid DNA vaccines is central to the success of these Types of Vaccines. Various strategies considered later in this chapter, have been developed which target vaccine delivery to DCs to enhance the immunogenicity of vaccine.

  13.3 Vaccines in current use in humans

  Current vaccines which have been approved for use in humans have proven efficacy as well as generating much information on the nature and mechanisms of vaccine-induced immunity in humans. These vaccines are of three main types: live-, attenuated-, whole-organism vaccines, inactivated vaccines and subunit vaccines. Examples of each are considered in this section.

  13.3.1 Live-, attenuated-vaccines

  These vaccines use strains of the pathogen that have been attenuated, but retain features of the human strain. Attenuated strains may arise naturally and are typically strains which cause disease in non-human hosts but are less virulent in humans, or pathogenic strains may be attenuated through laboratory manipulation, to make them safe for use in humans. Attenuation can be achieved through growing the pathogen under abnormal conditions for the pathogen, and screening the isolated strains for loss of virulence while retaining immunogenicity. This strategy can be a time-consuming process. The development of the BCG strain, isolated by Calmette and Guerin in the 1920s took 13 years of culturing M. bovis. BCG is the most widely-used vaccine and elicits both strong B and T cell responses. However, the efficacy of BCG is highly variable; this is due partly, perhaps, to variations between vaccine preparations due to the fact that BCG has never been cloned. The only other approved, live, bacterial vaccine is the attenuated Salmonella typhi

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  Biotechnology in Medical Sciences

  • Firdos Alam Khan(Author)
  • 2014(Publication Date)
  • CRC Press

    (Publisher)

  97 Chapter four: Virology and vaccines 4.8.1.5 Other vaccines Recently, various efforts have been made to make vaccines in different ways. An immune response can be achieved by introducing a protein subunit rather than introducing an inactivated or attenuated microorganism. Examples include the subunit vaccine against hepatitis B virus that is composed of only the surface proteins of the virus (previously extracted from the blood serum of chronically infected patients, but now produced by recombination of the viral genes into yeast), the virus-like particle (VLP) vaccine against HPV that is composed of the virus capsid protein, and the hemagglutinin and neuramini-dase subunits of the influenza virus. The vaccine can also be prepared by conjugating certain bacteria that have polysaccha-ride outer coats that are poorly immunogenic. Moreover, by connecting these outer coats to proteins, for example, toxins, the immune system can be led to recognize the polysaccha-ride as if it were a protein antigen. Interestingly, this approach is used in the Haemophilus influenzae type B vaccine development. Furthermore, by similar methods, dendritic cell vac-cines can also be made by combining dendritic cells with antigens in order to present the antigens to the body’s white blood cells, thus stimulating an immune reaction. These vac-cines have shown certain positive preliminary results for treating brain tumors. Moreover, vaccines can be monovalent or univalent or multivalent in nature. A monovalent vaccine can immunize against a single antigen or single microorganism, whereas a multivalent vaccine is developed to immunize against two or more strains of the same microorganism, or against two or more microorganisms, respectively. In certain cases, it has been observed that a monovalent vaccine may be preferable for rapidly developing a strong immune response.

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  Encyclopedia of Pharmaceutical Technology

  Volume 6

  • James Swarbrick(Author)
  • 2013(Publication Date)
  • CRC Press

    (Publisher)

  Unit–Vali dation Vaccines and Other Immunological Products Suresh K. Mittal Harm HogenEsch Department of Veterinary Pathobiology, Purdue University, West Lafayette, Indiana, U.S.A. Kinam Park Departments of Pharmaceutics and Biomedical Engineering, Purdue University, West Lafayette, Indiana, U.S.A. INTRODUCTION The concept of vaccination was introduced in the late 18th century by Edward Jenner when he used cowpox virus as a vaccine to protect humans against smallpox virus infections. This led to the develop-ment of vaccines over the next 2 centuries to provide protection against various bacterial and viral patho-gens. Undoubtedly, the effective vaccination against infectious diseases is the best method of reducing suffering of human and animals caused by viral, bacterial, and parasitic infections. Over the last 200 years, the technology of vaccine development and production has not changed significantly. This usually involves the use of either a killed pathogen combined with an adjuvant or a livepathogen with reduced virulence. Apart from the tremendous suc-cess of killed and attenuated virus vaccines over the years, many of such vaccines do not provide satisfactory protection, and there are a number of other disadvantages associated with these vaccines. Additionally, there are important pathogens against which attempts to develop effective vaccines using traditional approaches were unsuccessful. Various protective viral antigens (envelope and/or capsid proteins or glycoproteins and other viral proteins) and bacterial antigens (surface, internal, or fimbria proteins; bacterial polysaccharides; bacterial toxins; and other proteins involved in bacterial metabolism) have been identified as potential vaccine candidates. These protective antigens are used by various means to develop effective vaccines.

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