Vaccination

Vaccination is one of the most important medical developments in history. It has saved hundreds of millions of lives and led to the eradication of smallpox. In this lesson you will learn how vaccines work, what herd immunity is, and the benefits and limitations of vaccination programmes.

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What Is a Vaccine?

A vaccine is a preparation containing a dead or weakened (attenuated) pathogen, or parts of the pathogen (such as surface proteins or inactivated toxins).

When injected into the body, the vaccine stimulates the immune system to mount a response without causing the disease.

Key Point

The pathogen in the vaccine is either:

• Dead (killed) — cannot reproduce or cause disease
• Weakened (attenuated) — alive but significantly less dangerous; can reproduce slowly but cannot cause serious disease
• A fragment of the pathogen — e.g. surface antigens or inactivated toxins (toxoids)

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How Vaccination Works

The mechanism relies on the immune system's ability to form memory cells:

Step-by-Step Process

1. A vaccine containing dead, weakened, or parts of a pathogen is injected into the body
2. The antigens on the pathogen (or pathogen fragments) are detected by lymphocytes
3. The lymphocytes produce specific antibodies that bind to the antigens
4. Memory lymphocytes are formed and remain in the body for a long time
5. If the real, live pathogen enters the body in the future, the memory cells recognise its antigens immediately
6. Memory cells divide rapidly and produce antibodies quickly and in large quantities — this is the secondary immune response
7. The pathogen is destroyed before it can reproduce enough to cause symptoms
8. The person is said to be immune

Important Distinctions

• The vaccine does not contain the active, disease-causing pathogen — it cannot give you the disease
• The person does not become ill from the vaccine (though mild side effects like soreness at the injection site or a slight fever are normal — these are signs of the immune system responding)
• The vaccine gives the immune system a "practice run" so it is ready for the real thing

Exam tip: When explaining how vaccines work, always mention: (1) the vaccine contains dead/weakened pathogens or antigens, (2) the immune system produces antibodies, (3) memory cells are formed, and (4) these memory cells allow a rapid secondary response if the real pathogen is encountered. Step 3 (memory cells) is the most commonly missed point.

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Herd Immunity

Herd immunity occurs when a large enough proportion of a population is vaccinated (or immune) so that the pathogen cannot spread easily, even to those who are not vaccinated.

How It Works

• If most people in a community are immune (through vaccination), there are very few people who can be infected
• The pathogen cannot find enough susceptible hosts to spread through the population
• This protects people who cannot be vaccinated, such as:
  • Very young babies (too young for certain vaccines)
  • People who are immunocompromised (e.g. undergoing chemotherapy, organ transplant recipients, people with HIV)
  • People with allergies to vaccine components

The Threshold

• For highly contagious diseases, a very high percentage of the population must be vaccinated for herd immunity to work
• For measles, approximately 95% of the population needs to be vaccinated
• If vaccination rates drop below this threshold, herd immunity breaks down and outbreaks can occur

Exam tip: Herd immunity protects vulnerable people who cannot be vaccinated. This is a key benefit of high vaccination rates that you should include in any question about the advantages of vaccination programmes.

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Benefits of Vaccination

Benefit	Explanation
Prevents epidemics and pandemics	If enough people are vaccinated, the pathogen cannot spread through the population
Eradicated smallpox	The WHO declared smallpox eradicated in 1980 — the first (and so far only) human disease to be completely wiped out by vaccination
Near-eradication of polio	Polio cases have been reduced by over 99% worldwide since 1988 through vaccination campaigns
Protects vulnerable individuals	Through herd immunity, vaccination protects those who cannot be vaccinated
Cost-effective	Preventing disease through vaccination is far cheaper than treating the disease
Reduces antibiotic use	Preventing bacterial infections through vaccination reduces the need for antibiotics, slowing the development of antibiotic resistance

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Limitations of Vaccination

Limitation	Explanation
Vaccines don't always work	Some individuals may not develop full immunity even after vaccination (their immune system may not respond strongly enough)
Some people can't be vaccinated	Immunocompromised individuals may not be able to safely receive certain vaccines (especially live attenuated vaccines)
Pathogen mutation	Some pathogens mutate rapidly, changing their surface antigens. The antibodies from the old vaccine may not recognise the new strain. This is why the influenza (flu) vaccine must be updated every year.
Side effects	Most side effects are mild (soreness, mild fever), but rarely, more serious reactions can occur. The overall risk is extremely small compared to the risk of the disease itself.
Logistics and cost	Developing, distributing, and storing vaccines (some require cold chains) can be expensive and logistically challenging, especially in developing countries
Public mistrust	Misinformation about vaccines can reduce uptake, threatening herd immunity

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The MMR Vaccine Controversy

Background

In 1998, Andrew Wakefield published a paper in The Lancet medical journal claiming a link between the MMR vaccine (measles, mumps, rubella) and autism in children.

What Happened

• The paper caused widespread fear, and many parents refused to vaccinate their children
• MMR vaccination rates dropped significantly in the UK
• This led to outbreaks of measles — a disease that had been well controlled

The Truth