Vaccines- Introduction and Types

Vaccines- Introduction and Types

  • A vaccine is a medical preparation given to provide immunity from a disease.
  • Vaccines use a variety of different substances ranging from dead microorganisms to genetically engineered antigens to defend the body against potentially harmful microorganisms.
  • Effective vaccines change the immune system by promoting the development of antibodies that can quickly and effectively attack disease-causing microorganism when it enters the body, preventing disease development.
  • A vaccine may contain live attenuated or killed microorganisms or parts or products from them capable of stimulating a specific immune response comprised of protective antibodies and T cell immunity.
  • A vaccine should stimulate a sufficient number of memory T and B lymphocytes to yield effector T cells and antibody-producing B cells from memory cells.
  • Viral vaccine should also be able to stimulate high titers of neutralizing antibodies.
  • Injection of a vaccine into a nonimmune subject induces active immunity against the modified pathogens.
  • Vaccination is immunization against infectious disease through the administration of vaccines for the production of active (protective) immunity in humans or other animals.

Vaccines- Introduction and Types

There are 4 main types of vaccines:

  1. Live Attenuated vaccines (LAV)
  2. Inactivated vaccines (Killed Antigen)
  3. Subunit and Conjugate Vaccines (Purified Antigen)
  4. Toxoid vaccines (Inactivated Toxins)

A. Live Attenuated Vaccines

  • In some cases, microorganisms can be attenuated or disabled so that they lose their ability to cause significant disease (pathogenicity) but retain their capacity for transient growth within an inoculated host.
  • Some agents are naturally attenuated by virtue of their inability to cause disease in a given host, although they can immunize these individuals.
  • The first vaccine used by Jenner is of this type: vaccinia virus (cowpox) inoculation of humans confers immunity to smallpox but does not cause smallpox.
  • Attenuation can often be achieved by growing a pathogenic bacterium or virus for prolonged periods under abnormal culture conditions.
  • This selects mutants that are better suited for growth in the abnormal culture conditions than in the natural host.
  • For example, an attenuated strain of Mycobacterium bovis called Bacillus Calmette- GuĂ©rin (BCG) was developed by growing bovis on a medium containing increasing concentrations of bile.
  • After 13 years, this strain had adapted to growth in strong bile and had become sufficiently attenuated that it was suitable as a vaccine for tuberculosis.
  • Due to variable effectiveness and difficulties in follow-up monitoring, BCG is not used in the United States.
  • The Sabin form of the polio vaccine and the measles vaccine both consist of attenuated viral strains.

Examples:

  • Vaccinia (smallpox)
  • Measles, mumps, rubella (MMR combined vaccine)
  • Varicella (chickenpox)
  • Influenza (nasal spray)
  • Rotavirus
  • Zoster (shingles)
  • Yellow fever

B. Inactivated vaccines (Killed Antigen)

  • Another common means to make a pathogen safe for use in a vaccine is by treatment with heat or chemicals.
  • This kills the pathogen, making it incapable of replication, but still allows it to induce an immune response to at least some of the antigens contained within the organism.
  • It is critically important to maintain the structure of epitopes on surface antigens during inactivation.
  • Heat inactivation is often unsatisfactory because it causes extensive denaturation of proteins; thus, any epitopes that depend on higher orders of protein structure are likely to be altered significantly.
  • Chemical inactivation with formaldehyde or various alkylating agents has been successful.
  • The Salk polio vaccine is produced by formaldehyde inactivation of the poliovirus.

Examples:

  • Polio (IPV)
  • Hepatitis A
  • Rabies

C. Subunit and Conjugate Vaccines (Purified Antigen)

  • These subunit vaccines are composed of antigens purified from microbes which are usually administered with an adjuvant.
  • Vaccines composed of bacterial polysaccharide antigens are used against pneumococcus and Haemophilus influenzae.
  • Because polysaccharides are T-independent antigens, they tend to elicit low-affinity antibody responses and are poorly immunogenic in infants (who do not mount strong T cell-independent antibody responses).
  • High affinity antibody responses may be generated against polysaccharide antigens even in infants by coupling the polysaccharides to proteins to form conjugate vaccines.
  • These vaccines elicit helper T cells to simulate germinal center reactions, which would not occur with simple polysaccharide vaccines.
  • Such vaccines work like hapten-carrier conjugates and are a practical application of the principle of T-B cell cooperation.

Examples:

  • Hepatitis B
  • Influenza (injection)
  • Haemophilus influenzae type b (Hib)
  • Pertussis (part of DTaP combined immunization)
  • Pneumococcal
  • Meningococcal
  • Human papillomavirus (HPV)

D. Toxoid vaccines (Inactivated Toxins)

  • Toxoid vaccines use a toxin (harmful product) made by the germ that causes a disease.
  • They create immunity to the parts of the germ that cause a disease instead of the germ itself.
  • That means the immune response is targeted to the toxin instead of the whole germ.
  • Like some other types of vaccines, you may need booster shots to get ongoing protection against diseases.

Examples:

  • Diphtheria, tetanus (part of DTaP combined immunization)

Vaccines- Introduction and Types

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