Principles and Types of Vaccination

Principles of Vaccination

• Vaccination is the process of introducing a weakened or inactive form of a pathogen into the body to stimulate an immune response.
• Immunity is the body's ability to recognize and eliminate foreign substances.
• Immunization is the process of inducing immunity against a disease.
• Passive immunity is acquired by receiving pre-formed antibodies, either naturally (e.g., from mother to child) or artificially (e.g., by injection of antibodies).
• Active immunity is acquired by the body's own immune response to a pathogen, either through natural infection or vaccination.

Types of Vaccines

• Live attenuated vaccines: Weakened forms of a living pathogen.
  • Examples: Measles, mumps, rubella (MMR), varicella (chickenpox), influenza.
  • Microorganisms are weakened/attenuated, losing their ability to cause significant disease but retaining their capacity for transient growth within the inoculated host.
  • The first vaccine developed (by Jenner), inoculating humans with vaccinia (cowpox) to confer smallpox immunity.
  • Attenuation can be achieved by growing pathogenic bacteria or viruses in abnormal culture conditions to select better growth suited mutants.
  • Example: BCG is an attenuated strain of Mycobacterium bovis.
  • Live attenuated vaccines replicate in the host and induce both humoral and cell-mediated immunity.
  • Live attenuated vaccines require only a single immunization.
• Inactivated vaccines: Killed forms of a pathogen.
  • Examples: Inactivated polio, hepatitis A, influenza, rabies.
  • Pathogens are treated with heat or chemicals to kill it, making it incapable of replication but able to induce an immune response.
  • Maintaining structure of epitopes on surface antigens is important. Heat inactivation is often unsatisfactory because it denatures proteins. Chemical inactivation is more successful.
  • Examples: The Salk polio vaccine was produced by formaldehyde inactivation.
  • Killed vaccines often require repeated boosters as they don't replicate.
  • Produce predominantly humoral response and are less effective than attenuated vaccines in inducing cell-mediated immunity.
  • Increased safety compared to live attenuated vaccines.
• Subunit vaccines: Use specific antigenic parts of a pathogen.
  • Examples: Hepatitis B, some pneumococcal vaccines.
  • Contain only specific, purified macromolecules derived from the pathogen.
  • Contain only the necessary antigenic parts of the pathogen to elicit a protective immune response.
  • Common subunit vaccines include inactivated exotoxins, capsular polysaccharides/surface glycoproteins, and key recombinant proteins.
• Toxoid vaccines: Inactivated toxins produced by bacteria.
  • Examples: Tetanus, diphtheria.
  • Inactivated exotoxins are treated with heat or chemicals.
  • Unable to cause disease but stimulate the production of antitoxin antibodies, neutralizing their effects.
• Capsular polysaccharide vaccines: Purified capsular polysaccharides of bacteria.
  • Examples: Pneumococcal, meningococcal, Haemophilus influenzae type b (Hib).
  • The virulence of some bacteria depends on their antiphagocytic polysaccharide capsule.
  • Coating the capsule with antibodies/complement increases phagocytosis by macrophages and neutrophils.
• Recombinant vector vaccines: Use a modified pathogen as a vector to deliver antigens.
  • Examples: Some yellow fever vaccines.
  • Live attenuated vaccines that deliver antigens, but avoid reverting to pathogenic forms.
  • Individual genes encoding key antigens are introduced into attenuated viruses/bacteria, enabling replicating organisms to express pathogen genes, eliminating reversion potential.
• DNA vaccines: Introduce pathogen genes into cells to produce antigens.
  • Plasmid DNA encoding antigenic proteins is injected directly into muscle tissue.
  • DNA either integrates into chromosomal DNA or exists episomally.
  • Often taken up by dendritic cells/muscle cells in the injection area.
  • Induces protective immunity against various pathogens.
  • Follow-up booster shots or supplementary DNA may enhance immune response.
  • Advantages: no denaturation, exact antigen expression, no refrigeration needed, simultaneous manufacture of multiple vaccines, successful human trials underway.

Passive Immunity

• Homologous pooled human antibody (immune globulin): IgG antibody fraction from many donors. Used for prophylaxis of hepatitis A and measles.
• Homologous human hyperimmune globulins: Antibody products with high titers of specific antibodies. Used for hepatitis B, rabies, and tetanus.
• Heterologous hyperimmune serum (antitoxin): Produced in animals (e.g., horses). Contains antibodies against a specific antigen. Used for botulism and diphtheria.
• Monoclonal antibody: Produced from a single B cell clone. Contains antibodies against only one or closely related group of antigens. Used in cancer treatment, transplant rejection, and autoimmune disease treatment.

Active Immunity

• Stimulation of the immune system to produce antigen-specific antibody responses.
• Unlike passive immunity, active immunity lasts for extended periods, with persistence known as immunologic memory.