Vaccination, Immunity Types and Antibiotic Resistance

The final OCR topic in communicable diseases and immunity brings together vaccination, the four types of immunity, herd immunity, and the growing problem of antibiotic resistance. OCR specification 4.1.1 (h)–(j) requires you to understand how vaccines work, the differences between active/passive and natural/artificial immunity, the impact of herd immunity on epidemics, and how bacteria become resistant to antibiotics, with named examples.

Key Definitions:

• Vaccination — the deliberate exposure to antigenic material to produce an immune response and memory cells without causing disease.
• Herd immunity — the protection of unvaccinated individuals in a population where enough others are immune to prevent disease spread.
• Epidemic — an outbreak of disease affecting many people in a community at the same time.
• Pandemic — an epidemic that has spread across multiple countries or continents.
• Antibiotic resistance — the ability of bacteria to survive exposure to antibiotics that would normally kill them.

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Vaccination: Principle

A vaccine delivers antigen from a pathogen in a form that cannot cause serious disease. The immune system responds as if it were fighting the real pathogen: clonal selection and expansion occur, plasma cells produce antibodies, and memory B and T cells are formed. When the individual later meets the real pathogen, the secondary response is so fast and strong that the infection is cleared before it causes illness.

flowchart LR
    A[Vaccine administered] --> B[Antigen recognised by lymphocytes]
    B --> C[Clonal selection and expansion]
    C --> D[Plasma cells secrete antibodies]
    C --> E[Memory B and T cells formed]
    E --> F[Later exposure to real pathogen]
    F --> G[Rapid secondary response]
    G --> H[No illness]

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Types of Vaccine

Type	Mechanism	Examples
Live attenuated	Weakened live pathogen; multiplies but does not cause disease	MMR, BCG (TB), oral polio (Sabin)
Inactivated (killed)	Whole pathogen killed by heat or chemicals	Injected polio (Salk), hepatitis A, rabies
Subunit	Pure antigen or fragment of pathogen	Hepatitis B (surface antigen), HPV
Toxoid	Inactivated bacterial toxin	Tetanus, diphtheria
Conjugate	Polysaccharide antigen linked to a carrier protein	Hib, pneumococcal (PCV), meningococcal
mRNA	Lipid nanoparticles carrying mRNA coding for antigen	Pfizer-BioNTech and Moderna COVID-19
Viral vector	Harmless virus carrying pathogen gene	AstraZeneca COVID-19, Ebola (rVSV-ZEBOV)

Each type has advantages and disadvantages. Live attenuated vaccines produce strong, long-lasting immunity but are unsuitable for immunocompromised patients. Inactivated vaccines are safer but usually need boosters. mRNA vaccines can be developed quickly and scaled but are less stable and require cold storage.

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The Four Types of Immunity

Immunity can be classified along two axes: active/passive and natural/artificial. This gives four combinations.

1. Natural Active Immunity

The body encounters a pathogen naturally and mounts a full immune response, producing antibodies and memory cells. Provides long-lasting immunity.

• Example: having and recovering from measles.

2. Artificial Active Immunity

The body is deliberately exposed to antigen via a vaccine. The immune system mounts a response and produces memory cells. Provides long-lasting immunity, similar to natural active.

• Example: MMR vaccine, polio vaccine.

3. Natural Passive Immunity

Antibodies are transferred to an individual naturally without the individual's own immune system producing them.

• Example: IgG crosses the placenta from mother to fetus; IgA in breast milk protects the newborn.
• Short-lived, because no memory cells are made; the antibodies degrade over weeks to months.

4. Artificial Passive Immunity

Antibodies are injected into the individual from an external source.

• Example: anti-venom for snake bites; anti-tetanus immunoglobulin after injury; monoclonal antibodies against SARS-CoV-2.
• Also short-lived but acts immediately — crucial when there is no time for an active response (e.g., after a bite from a rabid animal, combined with active vaccination).

Summary Table

Type	Antibody source	Memory cells?	Duration
Natural active	Own body, after infection	Yes	Long
Artificial active	Own body, after vaccine	Yes	Long
Natural passive	Mother (placenta, breast milk)	No	Short (weeks–months)
Artificial passive	Injected antibodies	No	Short (days–weeks)

Exam Tip: The key divide is whether the recipient's own immune system makes the antibodies. If yes → active and long-lasting. If no → passive and short-lived. OCR loves this distinction.

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Herd Immunity

When a large fraction of a population is immune to a disease — whether through vaccination or past infection — the pathogen struggles to find new susceptible hosts. Chains of transmission break, and even unvaccinated individuals (newborns, immunocompromised people) gain protection. This is herd immunity.

The Herd Immunity Threshold

The threshold depends on the disease's basic reproduction number (R₀). Roughly:

Herd immunity threshold = 1 − (1/R₀)

• Measles (R₀ ≈ 15) needs ~95% immunity.
• Mumps (R₀ ≈ 10) needs ~90% immunity.
• Polio (R₀ ≈ 6) needs ~83% immunity.
• Seasonal flu (R₀ ≈ 1.3) needs ~25%.

If vaccination rates fall below these thresholds, outbreaks return — as happened with measles in the UK and US in the late 2010s after misinformation linking MMR to autism caused vaccination rates to drop.

Benefits

• Protects individuals who cannot be vaccinated (infants, immunocompromised).
• Eventually eradicates diseases (smallpox, 1980; rinderpest, 2011; polio, almost).

Limitations

• Works only for diseases with exclusively human reservoirs.
• Requires very high coverage for highly contagious diseases.
• Relies on sustained vaccination and surveillance.

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Epidemics and Pandemics