Vaccination, Immunity Types and Antibiotic Resistance

Spec Mapping — OCR H420 Module 4.1.1 — Communicable diseases, disease prevention and the immune system, content statement on vaccination, types of immunity (active/passive, natural/artificial), herd immunity, epidemics and pandemics, and antibiotic resistance with named examples (refer to the official OCR H420 specification document for exact wording). This lesson is the culminating synthesis of the module: how the immune biology of Lessons 6–9 translates into vaccination programmes that protect populations, and how the emergence of antibiotic resistance is now one of the gravest threats to modern medicine.

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. The historical arc is one of the great triumphs and one of the great looming failures of modern medicine. The triumphs: Edward Jenner's 1796 observation that milkmaids who had had cowpox were immune to smallpox led to the first vaccine and, eventually, to the global eradication of smallpox in 1980 — the only human infectious disease ever eradicated. Louis Pasteur developed attenuated vaccines against fowl cholera, anthrax and rabies in the 1880s, generalising vaccination beyond cross-species protection. The 20th century brought conjugate, subunit, toxoid and recombinant vaccines, and the early 2020s delivered the first widely-deployed mRNA vaccines against SARS-CoV-2 within a year of pathogen identification. The looming failure: Alexander Fleming's 1928 chance discovery of penicillin from Penicillium notatum mould launched the antibiotic era, but his 1945 Nobel speech warned explicitly that resistance would follow. He was right. MRSA, MDR-TB, carbapenem-resistant Enterobacteriaceae and vancomycin-resistant enterococci are now routine clinical problems, and the global pipeline of new antibiotics is slim. OCR specification 4.1.1 requires you to understand both the vaccination success and the antibiotic-resistance crisis at the level of cellular and population biology.

Key Definitions:

• Vaccination — the deliberate exposure of an individual to antigenic material from a pathogen, sufficient to elicit an adaptive immune response (and crucially, immunological memory) without causing serious disease.
• Herd immunity — the indirect protection of unvaccinated individuals in a population in which a sufficiently high fraction of others is immune, reducing R below 1 and breaking chains of transmission.
• Epidemic — a sudden increase in the number of cases of a disease above the expected baseline in a community.
• Pandemic — an epidemic that has spread across multiple countries or continents.
• Antibiotic resistance — the ability of bacteria to survive exposure to concentrations of an antibiotic that would normally inhibit or kill them, usually arising via mutation or horizontal gene transfer.
• Selective pressure — any environmental factor (here, the antibiotic) that preferentially allows the survival and reproduction of some genotypes over others.

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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 clonal expansion (Lesson 8) occur, plasma cells produce antibodies (Lesson 9), cytotoxic T cells are primed, and crucially memory B and T cells persist. 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.

The principle was first applied empirically by Edward Jenner in 1796: a young dairymaid, Sarah Nelmes, had cowpox lesions; Jenner inoculated the eight-year-old James Phipps with material from these lesions; six weeks later he challenged Phipps with smallpox material, and the child did not contract the disease. We now understand this in molecular terms: cowpox and smallpox (variola) viruses share enough surface antigen that anti-cowpox antibodies cross-react with smallpox. Jenner's empirical observation became the foundation of immunology, and the word vaccination itself derives from vacca, Latin for cow.

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 from a single or two-dose schedule but are unsuitable for immunocompromised patients and pregnant women (risk of reversion). Inactivated whole-pathogen vaccines are safe even for the immunocompromised but typically need multiple booster doses to maintain immunity. Subunit and toxoid vaccines allow precise targeting of protective epitopes but often need adjuvants (alum, AS01) to elicit a robust immune response. Conjugate vaccines rescue polysaccharide antigens (which alone are T-cell-independent and poorly immunogenic in young children) by attaching them to a carrier protein (tetanus toxoid, CRM197), recruiting T-cell help. mRNA vaccines can be designed within days of pathogen sequencing and manufactured at scale rapidly, but require cold chain storage and have only emerged as a major platform since 2020.

Vaccination — the antibody response over time

Memory cells persist between exposures

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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). The formula derives from the SIR model of mathematical epidemiology:

$\text{Herd immunity threshold} = 1 - \dfrac{1}{R_{0}}$Herd immunity threshold=1−R0​1​

• Measles (R₀ ≈ 15): threshold ≈ 1 − 1/15 ≈ 93 %, rounded up to 95 % to account for vaccine imperfection and heterogeneous mixing.
• Mumps (R₀ ≈ 10): threshold ≈ 90 %.
• Polio (R₀ ≈ 6): threshold ≈ 83 %.
• Smallpox (R₀ ≈ 5–7): threshold ≈ 80–86 %.
• Seasonal flu (R₀ ≈ 1.3): threshold ≈ 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 (in particular the discredited 1998 Lancet paper falsely linking the MMR vaccine to autism, retracted in 2010, with the lead author Andrew Wakefield struck off the UK medical register) caused MMR uptake to drop substantially in several countries and pockets of communities.

The herd-immunity calculation is sensitive to assumptions. It assumes random mixing in the population, equal vaccine efficacy across individuals, and lifelong immunity. In reality, mixing is non-random (children mix with other children, healthcare workers with patients), vaccine efficacy varies, and immunity often wanes — all of which raise the practically required coverage above the theoretical threshold. For waning immunity, booster doses (at school entry, adolescence, adulthood) are required to maintain population protection.

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

• Endemic — constantly present in a population at a baseline level (e.g., malaria in sub-Saharan Africa).
• Epidemic — a sudden increase above the expected level (e.g., cholera in Haiti after the 2010 earthquake).
• Pandemic — an epidemic spreading internationally (e.g., 1918 Spanish flu, 2009 H1N1, 2020 COVID-19).

Pandemics often involve new viral strains produced by antigenic shift (major change, as in influenza via reassortment in pigs/birds) or antigenic drift (gradual mutation of surface proteins). Because the population has no immunity to the new strain, rapid worldwide spread can occur. Vaccination campaigns, quarantine, and non-pharmaceutical interventions (masks, hand hygiene, lockdowns) are the main control measures.

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Antibiotic Resistance

Antibiotics are drugs that kill or inhibit the growth of bacteria. They work by interfering with bacterial-specific processes:

• Penicillin and cephalosporins — inhibit peptidoglycan cross-linking (cell wall synthesis).
• Tetracyclines and streptomycin — inhibit 30S ribosomal subunit (protein synthesis).
• Erythromycin — inhibits 50S ribosomal subunit.
• Rifampicin — inhibits bacterial RNA polymerase.
• Ciprofloxacin — inhibits DNA gyrase.

How Resistance Arises — Evolution by Natural Selection

flowchart TD
    A[Large bacterial population] --> B[Spontaneous random mutation in one cell]
    B --> C[Antibiotic applied as selective pressure]
    C --> D[Sensitive bacteria killed]
    C --> E[Resistant cell survives]
    E --> F[Resistant cell reproduces every 20 min]
    F --> G[Resistant clone dominates the population]
    G --> H[Plasmid conjugation spreads resistance to other species]
    H --> I[Multi-drug resistant superbug emerges]

Antibiotic resistance is the textbook example of evolution by natural selection operating on a microbial population:

1. Variation exists within any large bacterial population: spontaneous random mutations in genes affecting antibiotic targets, drug uptake or drug efflux produce a small fraction of cells with reduced susceptibility. Crucially, these mutations occur before the antibiotic is encountered — the antibiotic does not induce them. Luria and Delbrück's 1943 fluctuation test (Nobel 1969) was the experimental demonstration that mutations are pre-existing, not adaptive in the Lamarckian sense.
2. Selective pressure — when the antibiotic is applied, sensitive bacteria are killed, but resistant cells survive.
3. Inheritance — surviving cells reproduce (doubling every ~20 minutes in E. coli); their resistant offspring inherit the mutation; the resistant genotype rapidly dominates the population in a few hours.
4. Repeated exposure — further courses of the same or related antibiotics select for stacking resistance mutations, producing multidrug-resistant strains.

Mechanisms of resistance

Resistance can arise by many molecular mechanisms:

• Enzymatic destruction — β-lactamases hydrolyse the β-lactam ring of penicillins and cephalosporins; extended-spectrum β-lactamases (ESBLs) extend the substrate range; carbapenemases destroy even the last-line carbapenems.
• Target modification — MRSA's mecA gene encodes an altered penicillin-binding protein (PBP2a) that does not bind β-lactams; ribosomal mutations confer macrolide resistance; DNA gyrase mutations confer fluoroquinolone resistance.
• Efflux pumps — energy-driven transporters expel the antibiotic before it can act (tetracycline resistance is the classic case).
• Reduced uptake — loss of outer-membrane porins in gram-negative bacteria reduces β-lactam entry.
• Bypass pathways — Enterococcus species can swap D-alanyl-D-alanine for D-alanyl-D-lactate in their peptidoglycan, evading vancomycin binding (the basis of VRE).

Horizontal gene transfer

Resistance genes can spread horizontally between bacterial cells — including between different species — through three mechanisms:

• Conjugation — direct cell-to-cell transfer of a plasmid via a sex pilus; the dominant mechanism for spreading antibiotic-resistance plasmids in gram-negatives.
• Transformation — uptake of free DNA from the environment, releasing the resistance gene into a new host genome (Streptococcus pneumoniae is particularly competent for transformation).
• Transduction — bacteriophage-mediated transfer of bacterial DNA between cells (the mechanism that originally spread the SCCmec element in MRSA).