Vaccination stimulates the immune system to produce memory lymphocytes without the person experiencing the disease itself. If enough of a population is vaccinated, the pathogen can no longer spread effectively — a state called herd immunity — which indirectly protects individuals who cannot be vaccinated for medical reasons.
How does a vaccine work at the cellular level?
A vaccine contains a form of a pathogen that cannot cause the disease but does stimulate an immune response. This may be:
- Killed or inactivated pathogen — dead bacteria or viruses that still carry surface antigens.
- Weakened (attenuated) pathogen — a live but modified version that is too weak to cause illness in healthy people (e.g. the MMR vaccine).
- Toxoid — an inactivated toxin (e.g. tetanus vaccine).
- Subunit or protein antigen — just the surface protein of the pathogen, not the whole organism.
- mRNA vaccine — instructions that cause the body's own cells to produce a harmless pathogen protein, triggering an immune response (used in some COVID-19 vaccines).
When the vaccine is administered, the antigens are recognised as foreign by B lymphocytes. These white blood cells divide and differentiate into:
- Plasma cells — which secrete specific antibodies that bind to the antigens.
- Memory B cells — long-lived cells that persist in the bloodstream after the immune response subsides.
What role do memory cells play in vaccination?
The first time the immune system encounters an antigen — whether from infection or vaccination — the response is relatively slow, taking several days to produce significant antibody levels. This primary immune response is why people often feel ill before their immune system gains control of a natural infection.
After vaccination, memory B cells (and memory T cells) remain in the body for years or decades. If the same pathogen is encountered again, these memory cells mount a secondary immune response that is:
- Much faster — antibody production begins within hours rather than days.
- Much greater in magnitude — far more antibodies are produced.
- Effective enough to destroy the pathogen before symptoms develop.
This is why vaccinated people are protected: their immune system recognises the threat immediately and eliminates it before it can replicate to levels that cause disease.
What is the difference between active and passive immunity?
| Type | How acquired | Memory cells? | Duration |
|---|---|---|---|
| Active (natural) | Infection with a pathogen | Yes | Long-lasting (often lifelong) |
| Active (artificial) | Vaccination | Yes | Long-lasting (may need boosters) |
| Passive (natural) | Antibodies passed from mother to foetus via placenta, or in breast milk | No | Weeks to months |
| Passive (artificial) | Injection of antibodies (antiserum) | No | Weeks to months |
Active immunity involves the person's own immune system making memory cells — it takes time to develop but lasts. Passive immunity provides immediate protection (pre-made antibodies) but wears off as those antibodies are broken down, because no memory cells were formed.
What is herd immunity and how does it work?
Herd immunity occurs when a sufficiently high proportion of a population is immune to a disease — through vaccination or past infection — that the pathogen cannot find enough susceptible hosts to sustain its spread. Even individuals who are not immune gain indirect protection because the chain of transmission is broken.
The proportion of a population that must be immune is called the herd immunity threshold, and it varies by disease:
| Disease | Approximate herd immunity threshold |
|---|---|
| Measles | ~95% |
| Polio | ~80–85% |
| Mumps | ~85–90% |
| COVID-19 (original strain) | ~60–70% |
Measles requires a very high threshold because it spreads so easily — each infected person infects an average of 12–18 others in an unvaccinated population. For a disease to be eliminated, vaccination coverage must exceed the threshold consistently.
Who depends on herd immunity for protection?
Some people cannot be vaccinated or do not respond fully to vaccination:
- Newborn babies who are too young to receive certain vaccines.
- People with severe allergies to vaccine components.
- Immunocompromised individuals (e.g. those undergoing chemotherapy or with HIV) whose immune systems cannot mount a full response.
- A small percentage of vaccinated people for whom the vaccine does not produce lasting immunity (vaccine non-responders).
For these groups, herd immunity provided by the surrounding vaccinated population is their only protection against infectious disease. This is why high vaccination uptake is a public health responsibility, not just a personal choice.
What are the benefits and risks of vaccination programmes?
Benefits substantially outweigh risks for approved vaccines:
- Smallpox was completely eradicated worldwide by 1980 through vaccination — the first and so far only human disease to be eradicated.
- Polio has been eliminated from all but a handful of countries.
- In the UK, measles, mumps, rubella, diphtheria, tetanus, and polio are controlled by the childhood immunisation schedule.
All vaccines can cause mild side effects such as soreness at the injection site or a low-grade fever — these are signs of an immune response being mounted, not the disease itself. Serious adverse reactions are rare and are monitored continuously through the Yellow Card pharmacovigilance scheme (UK) and equivalent systems worldwide.
Frequently asked questions
How does vaccination produce immunity in GCSE biology?
A vaccine introduces antigens — surface molecules from a pathogen — into the body without causing the disease. The immune system responds by producing antibodies and memory B cells specific to those antigens. If the real pathogen is later encountered, the memory cells trigger a rapid secondary immune response, producing antibodies fast enough to neutralise the pathogen before illness develops. This long-lasting protection is called active artificial immunity.
What is herd immunity in GCSE biology?
Herd immunity occurs when enough people in a community are immune to a disease — through vaccination or prior infection — that the pathogen cannot spread efficiently from person to person. Even those who cannot be vaccinated, such as very young babies or immunocompromised individuals, are indirectly protected because the chain of transmission is broken. The percentage of the population that must be immune varies with how contagious the disease is.
What is the difference between active and passive immunity?
Active immunity involves the individual's own immune system producing memory cells and antibodies in response to antigens — either through natural infection or vaccination. It develops over days to weeks but lasts for years. Passive immunity provides immediate protection through antibodies made by someone else (transferred from mother to baby, or given as an injection), but it is temporary because no memory cells are formed and the borrowed antibodies break down within weeks to months.
Why do some vaccines require booster doses?
The level of antibodies in the blood tends to decline over time after vaccination. When antibody levels fall below a protective threshold, the person becomes susceptible to the disease again — even if memory cells are still present. A booster dose stimulates the memory cells to mount another secondary immune response, rapidly restoring high antibody levels. Some vaccines provide lifelong protection from a single course (e.g. MMR); others, like the flu vaccine, need annual renewal because the pathogen's antigens change.
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