Vaccine Types, Mechanisms, and Immunogenicity

Vaccines induce protective immunity by presenting the immune system with antigens in different forms, from whole inactivated or live-attenuated organisms to purified subunits, conjugates, and nucleic-acid platforms. This entry surveys the major vaccine types, how each engages the immune system, and what is meant by a vaccine's immunogenicity.

Definition

A vaccine is a preparation of antigenic material — a whole pathogen, a component of it, or instructions for the body to make an antigen — administered to induce adaptive immunity against an infectious agent. Immunogenicity is the capacity of a vaccine to elicit a measurable immune response, while a correlate of protection is the specific immune marker statistically associated with protection from disease.

Scope

The entry covers the principal vaccine platforms and the immunological mechanisms by which they elicit antibody and cellular responses and immunologic memory, together with the concept of immunogenicity and the correlates of protection that link immune responses to clinical benefit. It is a conceptual and educational overview of how vaccines work, not a guide to which vaccine to use or how to administer one.

Core questions

• What are the major vaccine platforms, and how do they differ in how they present antigen?
• How do live-attenuated, inactivated, subunit, conjugate, and nucleic-acid vaccines each engage the immune system?
• What roles do adjuvants and innate immune sensing play in shaping the response?
• What is immunogenicity, and how does it relate to a correlate of protection?
• Why do some vaccines confer durable, lifelong protection while others require boosting?

Key concepts

• Live-attenuated vaccines
• Inactivated (killed) vaccines
• Subunit, recombinant, and polysaccharide vaccines
• Conjugate vaccines
• Toxoid vaccines
• Viral-vector and nucleic-acid (mRNA/DNA) platforms
• Adjuvants and innate immune activation
• Humoral and cell-mediated responses
• Immunogenicity and correlates of protection
• Immunologic memory and durability

Mechanisms

Vaccines work by delivering antigen to the immune system in a form that triggers innate sensing, antigen presentation, and the priming of antigen-specific B and T lymphocytes, generating effector responses and long-lived memory (Pulendran & Ahmed, 2011). Platforms differ in how they achieve this: live-attenuated vaccines replicate transiently and broadly stimulate humoral and cellular immunity; inactivated and subunit vaccines present non-replicating antigen, often with an adjuvant to boost the response; conjugate vaccines couple a polysaccharide to a carrier protein to elicit T-cell help and memory in young children; toxoids neutralise toxin-mediated disease; and viral-vector and nucleic-acid platforms deliver genetic instructions so the recipient's cells transiently express antigen (Sadarangani, 2021). Antibody responses, including broadly neutralising antibodies for some pathogens, are central to protection for many vaccines (Corti & Lanzavecchia, 2013). The immune marker that tracks protection is the correlate of protection, which guides vaccine evaluation (Plotkin, 2010).

Clinical relevance

Understanding vaccine platforms and immunogenicity helps clinicians and public-health practitioners interpret why vaccines differ in dosing, durability, and the populations in which they work best. This entry is conceptual and educational and describes mechanisms of action; it does not recommend specific products or administration for any individual.

History

Vaccine development progressed from Jenner's and Pasteur's whole-organism approaches through twentieth-century inactivated, toxoid, polysaccharide and conjugate vaccines to recombinant subunit vaccines, and more recently to viral-vector and nucleic-acid platforms that came to prominence during the COVID-19 pandemic (Sadarangani, 2021). Across this history the unifying aim has been to present antigen in a form that safely elicits protective, durable immunity (Pulendran & Ahmed, 2011).

Key figures

• Bali Pulendran
• Rafi Ahmed
• Stanley A. Plotkin
• Antonio Lanzavecchia

Seminal works

• pulendran-2011
• plotkin-2010
• corti-2013

Frequently asked questions

What is the difference between a live-attenuated and an inactivated vaccine?

A live-attenuated vaccine contains a weakened but replicating form of the organism that broadly stimulates immunity, often with long-lasting protection. An inactivated vaccine contains a killed organism or its components that cannot replicate, frequently needs an adjuvant or several doses, and may require boosters.

What does immunogenicity mean?

Immunogenicity is a vaccine's ability to elicit a measurable immune response, such as antibodies or T-cell activity. When a particular response level is shown to track with protection from disease, it is called a correlate of protection (Plotkin, 2010).