Types of vaccines

Types of Vaccines - A Shot of Science

• Live-Attenuated: Weakened pathogen. Elicits robust, long-lasting humoral and cellular immunity. ⚠️ Contraindicated in pregnancy and immunocompromised states. (e.g., MMR, varicella, rotavirus).

• Inactivated (Killed): Pathogen destroyed by heat or chemicals. Induces mainly humoral immunity; requires boosters. (e.g., Polio [Salk], Hepatitis A).

• Subunit/Recombinant: Contains only specific antigenic components, not the whole organism. (e.g., Hepatitis B, HPV, acellular pertussis).

• Toxoid: Denatured bacterial toxin. Immune response targets the toxin. (e.g., Tetanus, Diphtheria).

⭐ Polysaccharide vaccines alone produce a T-cell-independent response (poor memory, weak in infants). Conjugation to a carrier protein allows for a robust, T-cell-dependent response with immunologic memory (e.g., H. influenzae type b vaccine).

Live-Attenuated Vaccines - Weakened Warriors

These vaccines use a weakened (attenuated) pathogen that replicates in the host, inducing a robust and long-lasting cell-mediated and humoral immunity that mimics natural infection.

• Examples: Measles, Mumps, Rubella (MMR), Varicella, Rotavirus, Sabin (oral) polio, intranasal influenza, BCG, Yellow fever.
• 📌 Mnemonic: "LIVE! One night only! See SMALL YELLOW CHICKENs get the SABIN and MMR!"
• ⚠️ Contraindications: Avoid in pregnancy and severe immunodeficiency.

⭐ Live vaccines given on different days must be separated by at least 4 weeks to avoid immune interference.

Inactivated Vaccines - Killed but Kicking

• Contain pathogens killed by heat or chemicals, rendering them non-infectious and unable to replicate.
• Primarily induce a humoral (B-cell) antibody response.
• Less immunogenic than live vaccines, thus requiring boosters and multiple doses.
• Safe for immunocompromised patients.

• Examples: 📌 R.I.P. Always
  • Rabies
  • Influenza (intramuscular)
  • Polio (Salk)
  • Hepatitis A

⭐ Because the pathogen cannot replicate, these vaccines induce a weaker cell-mediated immune response (Th1) and a stronger humoral response (Th2).

Subunit & Conjugate Vaccines - Pieces of the Puzzle

• Uses only specific antigenic components (e.g., proteins, polysaccharides) of a pathogen, not the whole organism, enhancing safety.

• Subunit/Recombinant Vaccines:

  • Contain purified pathogen proteins.
  • Examples: Hepatitis B (HBsAg), HPV, acellular pertussis.

• Polysaccharide vs. Conjugate:

  • Polysaccharide (e.g., PPSV23): Use capsule polysaccharides. Elicit a T-cell independent response; less effective in infants <2 years.
  • Conjugate (e.g., PCV, Hib, meningococcal): Polysaccharide is conjugated to a carrier protein, eliciting a robust, T-cell dependent response with memory.

⭐ Conjugate vaccines overcome the poor immunogenicity of pure polysaccharide antigens in infants by recruiting T-helper cells, enabling class switching (IgM → IgG) and immunologic memory.

Toxoid & Next-Gen Vaccines - Smart Shots

• Toxoid Vaccines

  • Mechanism: Inactivated bacterial toxin (toxoid) is injected, prompting the immune system to produce antibodies against the toxin itself, not the organism.
  • Examples: Tetanus, Diphtheria.

• mRNA & Viral Vector Vaccines

  • Mechanism: Deliver genetic code (mRNA or DNA) instructing host cells to produce a specific antigen, which then triggers a robust immune response.
  • Examples: COVID-19 vaccines.

⭐ Unlike traditional vaccines, mRNA platforms can be rapidly adapted to target new variants by simply changing the encoded genetic sequence.

High‑Yield Points - ⚡ Biggest Takeaways

• Live attenuated vaccines induce robust humoral and cell-mediated immunity but are contraindicated in immunocompromised patients and pregnancy.
• Killed-inactivated vaccines generate a weaker, primarily humoral (antibody) response and often require boosters.
• Subunit, recombinant, and polysaccharide vaccines are acellular and contain only the desired antigenic components.
• Toxoid vaccines contain inactivated toxins, inducing antibodies against the toxin, not the organism.
• Conjugate vaccines link a polysaccharide to a protein, enhancing the immune response in infants.