The 90-second version
- Your immune system learns to fight pathogens by recognising specific shapes called antigens on their surface
- A vaccine introduces your immune system to an antigen without the dangerous pathogen itself
- Your body mounts an immune response — producing antibodies and specialised cells — then stores that response as memory
- If the real pathogen ever enters your body, memory cells recognise it instantly and destroy it before it can cause serious illness
- Modern vaccines use different delivery methods: weakened or killed virus, protein subunits, or mRNA instructions to make your cells produce the antigen
- At high enough vaccination rates, even unvaccinated people are protected because the pathogen can’t find enough susceptible hosts — this is herd immunity
Your immune system: the two layers
To understand vaccines, you first need the immune system’s two-tier architecture.
Innate immunity is your body’s immediate, non-specific response. When bacteria enter a wound, innate immune cells flood in within minutes, causing inflammation and trying to kill everything unfamiliar. It doesn’t distinguish between different threats — it just reacts.
Adaptive immunity is the specialist layer. It takes 7–14 days to fully activate, but it creates precise, targeted defences against a specific threat — and crucially, it remembers it for years or decades.
Vaccines train the adaptive immune system. This is why vaccines require time to work — you need those 7–14 days for your adaptive response to fully develop.
The key players: B cells, T cells, antigens, antibodies
Antigens are the molecular shapes — usually proteins on the surface of a virus or bacterium — that your immune system uses to identify a threat. Think of them as the pathogen’s name tag.
B lymphocytes (B cells) are antibody factories. When they encounter an antigen that matches their receptor, they activate and start mass-producing antibodies — Y-shaped proteins that bind to that specific antigen. Antibodies can neutralise a pathogen directly (by blocking the part it uses to enter cells) or tag it for destruction by other immune cells.
T lymphocytes have two main roles:
- Cytotoxic T cells (CD8+): Kill infected cells directly by recognising antigen fragments displayed on the cell’s surface
- Helper T cells (CD4+): Orchestrate the immune response, activating B cells and other T cells
Memory cells: After the initial response winds down, a subset of B and T cells persist for years or decades as memory cells. They’re the fast-response team for future encounters.
How a vaccine triggers this system
A vaccine delivers an antigen (or instructions to make one) into your body without the actual danger of infection.
Step by step:
- Injection: The vaccine antigen is recognised by the innate immune system as foreign (especially if adjuvants are present — chemicals that act as “danger signals”)
- Antigen presentation: Dendritic cells capture the antigen and migrate to lymph nodes, displaying antigen fragments on their surface
- B and T cell activation: The right B cells and T cells — those whose receptors match the displayed antigen — are triggered and start multiplying rapidly
- Antibody production: Activated B cells produce millions of antigen-specific antibodies
- Memory formation: After the threat is cleared, memory B and T cells remain, on standby for decades
When the real pathogen arrives years later, memory cells recognise it within hours (not 14 days), mount an overwhelming response, and eliminate the infection before it can replicate enough to cause serious illness.
Types of vaccines
Different vaccines deliver the antigen (or antigen instructions) via different mechanisms:
Live-attenuated vaccines: A weakened (but alive) version of the pathogen. Produces strong, long-lasting immunity — often a single dose. Risk: very rarely can revert to virulence, problematic for immunocompromised people. Examples: MMR, yellow fever, varicella (chickenpox).
Inactivated vaccines: The pathogen is killed before injection. Safer but often requires multiple doses and boosters. Examples: polio (IPV), flu jab (some types), hepatitis A.
Protein subunit vaccines: Just the antigen (a specific protein from the pathogen’s surface), not the whole pathogen. Very safe, no risk of infection. Examples: Hepatitis B, HPV, some COVID-19 vaccines (Novavax).
mRNA vaccines: Genetic instructions (mRNA) encoding the antigen are injected. Your own cells read the instructions and temporarily produce the antigen, triggering an immune response. The mRNA degrades within days — it never enters the nucleus and can’t affect your DNA. Examples: Pfizer/BioNTech and Moderna COVID-19 vaccines.
Viral vector vaccines: A harmless virus (like adenovirus) carries DNA encoding the antigen into your cells. Examples: AstraZeneca COVID-19, Ebola vaccines.
Herd immunity: the mathematics of protection
When a large enough proportion of a population is immune, a pathogen can’t spread efficiently — even susceptible individuals gain indirect protection because the virus runs out of hosts.
The herd immunity threshold depends on how contagious the disease is (measured by R₀, the basic reproduction number):
| Disease | R₀ | Herd immunity threshold |
|---|---|---|
| Measles | 12–18 | ~92–95% |
| Polio | 5–7 | ~80–85% |
| COVID-19 (original) | 2–3 | ~60–67% |
| Influenza | 2–3 | ~50–67% |
Measles requires ~95% vaccination to maintain herd immunity — which is why outbreaks occur quickly when vaccination rates drop even slightly.
The mental model: a vaccine is a fire drill for your immune system
Imagine your immune system is a fire department that’s never seen a fire before. A vaccine is a carefully staged fire drill — you get smoke, the alarm sounds, the crew deploys and puts it out. But crucially, the crew now knows exactly what a fire looks like, where the equipment is, and the fastest routes.
When the real fire starts, the response is immediate and overwhelming. Without the drill, that 14-day mobilisation window is where the disease wins.
Common misconceptions
“Vaccines can give you the disease they prevent.” Inactivated and subunit vaccines contain no live pathogen — this is biologically impossible. Live-attenuated vaccines can theoretically cause mild disease in severely immunocompromised people, but this is extremely rare.
“Natural immunity is better than vaccine-induced immunity.” Sometimes, but the tradeoff is getting the disease first — which can be serious or fatal. mRNA COVID vaccines produced antibody levels comparable to or higher than natural infection in many studies. For diseases like HPV and tetanus, vaccines produce better immunity than natural infection.
“If I’m healthy, I don’t need vaccines — I’ll fight it off naturally.” Many vaccine-preventable diseases (measles, whooping cough, tetanus) are dangerous even for healthy adults. And vaccinating healthy people protects those who can’t be vaccinated — newborns, the immunocompromised, the elderly.
“The immune system can’t handle too many vaccines at once.” Your immune system handles millions of antigens every day from the environment. The combined childhood vaccine schedule introduces a tiny fraction of that. There is no evidence of immune system overload from vaccination schedules.