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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.
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]
| 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.
Immunity can be classified along two axes: active/passive and natural/artificial. This gives four combinations.
The body encounters a pathogen naturally and mounts a full immune response, producing antibodies and memory cells. Provides long-lasting 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.
Antibodies are transferred to an individual naturally without the individual's own immune system producing them.
Antibodies are injected into the individual from an external source.
| 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.
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 threshold depends on the disease's basic reproduction number (R₀). Roughly:
Herd immunity threshold = 1 − (1/R₀)
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.
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