How Vaccines Work

Intro

Vaccines train the immune system to recognise and fight infections without causing the full disease. They are one of the most effective public health interventions, preventing millions of deaths each year and helping eradicate or control deadly illnesses such as smallpox, polio, and measles.

Understanding how they work requires a look at both the immune system and the broader role of vaccination in public health.

Key Points

Immunity basics: Vaccines mimic infection to build protective memory without serious illness.
Innate vs adaptive: The innate system responds first; vaccines build adaptive memory (B cells, T cells).
Types of vaccines: Live attenuated, inactivated, subunit, conjugate, mRNA, and vector-based.
Adjuvants: Ingredients like aluminum salts boost response in some vaccines.
Herd immunity: When enough people are immune, diseases struggle to spread.
Surveillance: Safety is tracked continuously worldwide.
Myths: Claims such as a link to autism have been repeatedly debunked.

Background: The Immune System

Innate immunity: The body’s immediate, non-specific defense (skin, mucosa, macrophages, interferons).
Adaptive immunity: A targeted response involving B cells (antibodies) and T cells (killer and helper cells).
Immunological memory: After infection or vaccination, memory B and T cells persist, enabling faster, stronger responses to future exposure.

How Vaccines Train Immunity

Antigen introduction: A harmless part of the pathogen (protein, sugar, RNA instructions) is introduced.
Immune recognition: Antigen-presenting cells process the antigen and show it to T cells.
Activation: B cells produce antibodies; T cells coordinate and destroy infected cells.
Memory formation: Memory cells remain ready to respond rapidly on re-exposure.

Types of Vaccines

Live attenuated: Weakened pathogen (e.g., measles, yellow fever). Strong, durable immunity; not for immunocompromised.
Inactivated: Killed pathogens (e.g., polio [IPV], hepatitis A). Safer, may need boosters.
Subunit/conjugate: Specific proteins or sugars (e.g., HPV, Hib). Require adjuvants.
mRNA vaccines: Provide genetic instructions for antigen production (e.g., COVID-19). Fast to design, highly effective.
Viral vector: Harmless carrier virus delivers genetic instructions (e.g., Ebola, adenoviral COVID vaccines).

Role of Adjuvants

Adjuvants (such as aluminum salts, MF59, AS04) boost immune responses by stimulating innate sensors, prolonging antigen exposure, and improving adaptive immunity. They are safe, well-studied, and carefully regulated.

Herd Immunity

When a high proportion of the population is immune, transmission chains are disrupted.
Thresholds vary: measles (~95%), polio (~80–85%), COVID-19 (variable, higher due to evolving variants).
Herd immunity protects vulnerable people who cannot be vaccinated, such as infants and the immunocompromised.

Risks / Benefits

Benefits: Vaccines prevent disease, disability, and death, with benefits vastly outweighing risks.
Risks: Mild side effects (soreness, fever, fatigue) are common. Severe adverse events are extremely rare and are closely monitored.
Balance: The risk of vaccine-preventable disease is far greater than the risk of vaccination.

FAQ

Q: Can vaccines give me the disease?
A: Live attenuated vaccines very rarely revert, but licensed products are safe. Inactivated, subunit, and mRNA vaccines cannot cause infection.

Q: How long does immunity last?
A: It varies. Some (measles, yellow fever) give lifelong protection; others (influenza, pertussis) require boosters.

Q: Why are boosters needed?
A: To counter waning immunity or evolving pathogens.

Q: Do vaccines always prevent infection?
A: Not always. Some prevent severe illness and transmission even if mild infection occurs.