How Vaccines Work: Immunology, Ingredients, and Safety

Core Principle

Vaccines train the adaptive immune system to recognize a pathogen without causing disease, generating memory for rapid future response.

Antigen Types

Platform	Examples	Mechanism
Live attenuated	MMR, varicella, rotavirus, LAIV	Replicate weakly; robust cellular + humoral immunity
Inactivated/killed	IPV, hepatitis A, rabies	Cannot replicate; mainly humoral; adjuvant needed
Subunit/recombinant	Hepatitis B, HPV, Novavax COVID-19	Purified protein; safe, focused response
Toxoid	Diphtheria, tetanus	Inactivated toxin → antitoxin immunity
Viral vector	AstraZeneca, J&J, Sputnik V COVID-19	Adenovirus delivers transgene; strong T-cell response
mRNA	Pfizer/BioNTech, Moderna COVID-19	Host cells produce antigen; no genome integration

Key Excipients

• Adjuvants: Aluminum salts (alum), AS01, MF59, CpG — enhance immunogenicity.
• Stabilizers: Sugars, gelatin — protect during storage.
• Preservatives: 2-phenoxyethanol, thimerosal (multi-dose vials only) — prevent contamination.
• Buffers, surfactants, residuals: pH control, manufacturing traces (formaldehyde, antibiotics, egg protein).

Immune Response Timeline

1. Injection → antigen uptake by APCs.
2. Lymph node priming: T/B activation (days 3–7).
3. Germinal center reaction: affinity maturation, class switch (weeks).
4. Plasma cells → antibodies; memory B/T cells persist years-decades.

Safety

Rigorous phased trials (I–III), post-marketing surveillance (VAERS, VSD, PRISM). Benefits vastly outweigh risks for licensed vaccines.