How Vaccines Work: Immune Training & Herd Immunity

Vaccination: Mechanism, Types & Population Impact

Vaccines are among the most effective public health interventions, preventing 3.5–5 million deaths annually (WHO).

Immunological Mechanism

1. Antigen presentation: Vaccine delivers pathogen component (protein, polysaccharide, mRNA, viral vector)
2. Innate activation: Adjuvants (alum, AS01, TLR agonists) activate dendritic cells
3. Adaptive priming:
   • B cells → plasma cells (antibodies) + memory B cells
   • CD4+ T helper cells (Th1/Th2/Tfh) → cytokine support, class switching
   • CD8+ cytotoxic T cells (intracellular pathogens)
4. Memory formation: Long-lived plasma cells (bone marrow) + memory B/T cells
5. Upon exposure: Rapid anamnestic response (days vs. weeks) → neutralization before disease establishment

Vaccine Platforms

Herd Immunity (Community Protection)

• Threshold = 1 – 1/R₀ (R₀ = basic reproduction number)
• Measles (R₀ 12–18): 92–95% coverage needed
• Polio (R₀ 5–7): 80–86%
• Protects: Infants <6 mo, immunocompromised, vaccine non-responders, medically exempt

Safety & Monitoring

• Pre-licensure: Phase I/II/III trials (10⁴–10⁵ subjects)
• Post-marketing: VAERS, VSD, PRISM, CISA (US); EudraVigilance (EU)
• Rare adverse events detected only after millions of doses (e.g., VITT with adenovector COVID vaccines)
• Benefit-risk continuously reassessed

Historical Impact

• Smallpox: Eradicated 1980 (only human disease eradicated)
• Polio: 99.9% reduction; endemic in 2 countries (2024)
• Measles: 73% mortality drop 2000–2018 (but resurgences with coverage gaps)
• Hib: >99% reduction in invasive disease

Vaccination is a shared responsibility: Individual protection + community solidarity = disease control.

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Type	Examples	Mechanism	Pros/Cons
Live attenuated	MMR, Varicella, Rotavirus, Yellow fever	Replication-competent, weakened	Strong, durable immunity; contraindicated in immunocompromised
Inactivated/killed	IPV, Hepatitis A, Rabies, Influenza (IIV)	Non-replicating, whole pathogen	Safer; weaker immunity, needs boosters/adjuvant
Subunit/recombinant	Hepatitis B, HPV, Pertussis (aP), Shingles (RZV)	Purified protein/polysaccharide	Precise, safe; needs adjuvant
Toxoid	Diphtheria, Tetanus	Inactivated toxin	Targets toxin, not organism
Conjugate	Hib, Pneumococcal (PCV), Meningococcal	Polysaccharide + protein carrier	Overcomes T-independent limitation in infants
mRNA	COVID-19 (Pfizer/Moderna)	Host cells produce antigen	Rapid development, strong Th1/CD8; cold chain
Viral vector	COVID-19 (AstraZeneca, J&J), Ebola	Non-replicating vector delivers gene	Strong cellular immunity; pre-existing vector immunity