Introduction to vaccines

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Headshot of Joanna Breems, MD, FACP · Clinical Assistant Professor
Joanna Breems
MD, FACP · Clinical Assistant Professor
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Table of Contents
High-yield summary

Types of vaccines 

  • Live Attenuated: Replicates, broad immune response, contraindicated in immunocompromised and pregnancy 

  • Inactivated: No replication, safer for immunocompromised and pregnancy 

  • Polysaccharide and Conjugate: Capsule-targeted, age-dependent response, T-cell independent/dependent 

  • Protein Subunit: Toxoids, VLPs, recombinant proteins 

  • Viral Vector: Recombinant expression in vivo, adenovirus/VSV 

  • Nucleic Acid: mRNA/DNA vaccines, e.g., COVID-19 vaccines 

 

Immunology and vaccine mechanisms 

  • Active Immunity: Exposure to antigen → immune response → memory (e.g., vaccines). 

  • Passive Immunity: Transfer of antibodies (e.g., maternal IgG, IVIG); no memory formed. 

  • Live Attenuated Vaccines: Pathogen with reduced/minimal virulence replicates 

    • Strong, long-lasting immunity. 

    • May revert to virulence in immunocompromised. 

    • Examples: MMR, Varicella, Yellow Fever, Intranasal Influenza. 

  • Inactivated Vaccines: No replication of pathogen  

    • Safer, but weaker immune response. 

    • Often require boosters. 

    • Examples: Polio (IPV), Hepatitis A, Rabies. 

  • Polysaccharide vs. Conjugate Vaccines 

  • Polysaccharide Vaccines:  

    • T-cell independent. 

    • Poor response in children <2 years. 

    • No memory formation. 

  • Conjugate Vaccines:  

    • Polysaccharide linked to protein → T-cell dependent. 

    • More effective in infants. 

    • Examples: Hib, PCV13, Meningococcal conjugate. 

  • mRNA Vaccines (e.g., Pfizer, Moderna):  

    • Encodes mRNA to make target protein → host cells produce antigen. 

    • Rapid development, no risk of infection. 

  •  Viral Vector Vaccines (e.g., J&J, AstraZeneca):  

    • DNA encoding antigen delivered via harmless virus. 

    • May be replication competent/incompetent. 

  • Adjuvants 

    • Enhance immune response but are not themselves part of the immunogen. 

    • Common types:  

      • Aluminum salts: traditional adjuvant. 

      • MF59 (squalene): used in Fluad. 

      • CpG motifs: used in Heplisav-B. 

  • Vaccine Administration 

    • Routes: IM, SQ, oral, intranasal 

    • Maternal immunization: Tdap, influenza, COVID, RSV 

    • Placental antibody transfer for neonatal protection 

 

Contraindications and precautions 

  • Live Vaccines: Contraindicated in pregnancy and immunocompromised. 

  • Egg Allergy: Precaution for some influenza vaccines. 

  • Recent IVIG or blood products: May interfere with live vaccine efficacy. 

  • Precautions: Neurologic disorders/h/o of Guillain-Barré 

 

Vaccine safety and development 

  • Pre-licensure: Phases 1–3, investigate safety and efficacy 

  • Post-licensure: Surveillance, effectiveness, impact 

  • Contraindications: Anaphylaxis, pregnancy, immunodeficiency 

  • Precautions: Neurologic disorders, recent blood products 

 

Public health and vaccine hesitancy 

  • Goals: Distinguish between Individual vs. Population levels 

  •  Prevent infection, reduce transmission, eliminate disease 

  • Challenges: Misinformation, safety concerns, social media 

  • Strategies: Education, trusted sources, community engagement 

 

USMLE tips 

  • Live vaccines contraindicated in immunocompromise and pregnancy 

  • Know examples of each vaccine type 

  • Understand adjuvant mechanisms 

  • Differentiate active vs passive immunity 

Learning goals

  1. Compare and contrast the types of vaccines—live, killed/inactivated, polysaccharide, protein, mRNA—in terms of composition and how each type is generated
  2. Describe the contraindications and precautions for the different types of vaccines (live, killed/inactivated, polysaccharide, protein, mRNA)
  3. Compare and contrast how the immune system generates a productive antibody response to protein antigens versus carbohydrate antigens and how this affects vaccine response and development
  4. Describe the goals of vaccine programs as they pertain to individual and population health

Study materials

These materials are not required; they are supplementary to large-group session. They are intended as a curated guide to content focused on the learning objectives. There are both textbook and video resources for this session for students to use per their preference. For each reference, I have designated the learning goal addressed with a learning goal icon and and number.

Click the  book icons below to go to the library resources listed.