Types of Vaccines

Overview of Vaccine Types - Immune Kickstarters

• Principle: Stimulate adaptive immunity (B-cells, T-cells, memory) to a pathogen without causing disease. Mimics natural infection.
• Broad Categories:
  • Live-attenuated (e.g., MMR, BCG)
  • Inactivated/Killed (e.g., Salk Polio, Rabies)
  • Subunit (Protein, Polysaccharide, Conjugate) (e.g., Hep B)
  • Toxoid (e.g., Tetanus, Diphtheria)
  • Viral Vector (Recombinant) (e.g., some COVID-19)
  • Nucleic Acid (mRNA, DNA) (e.g., some COVID-19)

⭐ Live attenuated vaccines generally induce robust, long-lasting immunity (cellular & humoral) often with a single dose.

Live Attenuated Vaccines - Weakened Warriors

• Contain weakened (attenuated) live organisms; mimic natural infection to induce robust immunity.
• Immunity: Potent, long-lasting (humoral & cellular). Often lifelong with 1-2 doses (except oral).
• Advantages: Strong immune response, fewer doses needed.
• Disadvantages:
  • Potential reversion to virulence (rare).
  • ⚠️ Contraindicated: Immunocompromised individuals, pregnancy (most).
  • Circulating antibody interference.
  • Strict cold chain essential.
• 📌 Mnemonic: BOY TRICS GETS MMR (BCG, OPV, Yellow fever, Typhoid oral, Rotavirus, Influenza intranasal, Chickenpox, Smallpox, MMR).

⭐ OPV (Sabin) provides strong intestinal immunity (IgA) but carries a risk of Vaccine-Associated Paralytic Poliomyelitis (VAPP) (approx. 1 in 2.7 million first doses).

Inactivated (Killed) Vaccines - Neutralized Foes

• Whole bacteria/viruses killed by heat/chemicals (e.g., formalin, β-propiolactone); non-infectious.
• Key Features:
  • Cannot replicate or cause disease; no reversion to virulence.
  • Safer profile, suitable for immunocompromised individuals.
• Immune Response:
  • Mainly humoral (antibody) immunity; less cell-mediated response.
  • Less immunogenic than live vaccines.
  • Requires multiple doses (priming, boosters) & often adjuvants.
• Examples:
  • Viral: Influenza (shot), Polio (Salk/IPV), Rabies, Hepatitis A.
  • Bacterial: Pertussis (wP), Typhoid (parenteral), Cholera.

⭐ Salk vaccine (IPV), an inactivated polio vaccine, prevents poliomyelitis without risk of vaccine-associated paralytic polio (VAPP).

Component Vaccines - Purified Parts

• Uses specific antigenic components of pathogens, not whole organisms.
• Advantages: ↓ reactogenicity, ↑ safety, suitable for immunocompromised (if not live).

• Polysaccharide Vaccines
  • Capsular polysaccharides (e.g., Pneumococcal - PPSV23, Vi Typhoid).
  • T-cell independent response; poor in <2 yrs age.
• Conjugate Vaccines
  • Polysaccharide + protein carrier (e.g., Hib, Pneumococcal - PCV13, MenC).
  • T-cell dependent response; effective in infants.
• Recombinant Protein Vaccines
  • Gene for antigen inserted into host (e.g., yeast for Hep B surface Ag - HBsAg; HPV vaccine - L1 protein).
• Toxoid Vaccines
  • Inactivated bacterial toxins (e.g., Tetanus, Diphtheria).
  • Immunity to toxin, not organism.

⭐ Hepatitis B vaccine is a classic example of a recombinant subunit vaccine, produced by inserting the gene for HBsAg into yeast cells (e.g., Saccharomyces cerevisiae).

Newer Vaccine Technologies - Cutting-Edge Shields

• mRNA Vaccines: mRNA (antigen-coding) → host cell production → immunity.
  • Rapid development. E.g., Pfizer, Moderna (COVID-19).
• Viral Vector Vaccines: Harmless virus delivers pathogen genes.
  • E.g., AstraZeneca (Adenovirus), rVSV-ZEBOV (Ebola).
• DNA Vaccines: Plasmid DNA (antigen gene) → host cell expression.
  • Strong T-cell response.
• VLPs (Virus-Like Particles): Non-infectious viral protein shells.
  • Highly immunogenic. E.g., HPV.

⭐ mRNA vaccines: rapid development, potent humoral & cellular immunity.

High‑Yield Points - ⚡ Biggest Takeaways

• Live attenuated: Strongest immunity (IgA/IgG), risk reversion; contraindicated: immunocompromised (BCG, OPV, MMR).
• Killed/Inactivated: Safer, humoral immunity, need boosters (IPV, Rabies, Influenza shot).
• Toxoids: Inactivated bacterial toxins (Tetanus, Diphtheria), form antitoxin antibodies.
• Subunit/Acellular: Use specific purified antigens (Hepatitis B, acellular Pertussis).
• Conjugate: Link polysaccharide to protein carrier for T-cell response in infants (PCV, Hib).
• Recombinant vaccines: Produced by genetic engineering (e.g., Hepatitis B, HPV).
• mRNA/DNA vaccines: Deliver genetic material for host cells to synthesize antigens.