Explain how vaccines work, the role of herd immunity, and the development and implications of antibiotic resistance for human health

What this dot point is asking

QCAA expects you to explain how a vaccine produces immunity, what herd immunity is and what threshold it requires, and how antibiotic resistance arises through mutation and selection. You should be able to interpret a primary-secondary antibody response graph.

The answer

Vaccines and antibiotics are two of the most powerful tools of modern medicine. Vaccines exploit the adaptive immune system; antibiotics directly kill or inhibit bacteria. Both work, and both face challenges from biological reality (variable adaptive responses, evolving resistance).

How vaccines work

A vaccine introduces antigens from a pathogen in a form that cannot cause serious disease. The immune system treats the antigens as if they were the real pathogen and mounts a primary adaptive response (see innate and adaptive immunity):

1. Antigen-presenting cells take up the vaccine antigen and display it on MHC molecules.
2. Helper T cells specific to the antigen are activated.
3. B cells specific to the antigen are activated, undergo clonal expansion, and differentiate into plasma cells (producing antibodies) and memory B cells.
4. Cytotoxic T cells are activated and produce memory T cells.

The primary response is mild because the vaccine antigen cannot replicate or cause harm. The key outputs are the memory B and T cells, which can persist for years to a lifetime.

When the immunised person later encounters the real pathogen, memory cells trigger a secondary response that is faster, larger and longer-lasting than the primary response. The pathogen is usually cleared before symptoms appear.

Types of vaccine

• Live attenuated. Weakened live pathogen that still replicates a little. Strong, long-lasting immunity. Examples: MMR (measles, mumps, rubella), oral polio, varicella, BCG.
• Inactivated (killed). Pathogen killed by heat or chemicals; cannot replicate. Multiple doses or boosters often needed. Examples: hepatitis A, rabies, some flu vaccines.
• Subunit, recombinant or conjugate. Only specific antigens (proteins or polysaccharides) from the pathogen. Very safe. Examples: hepatitis B (recombinant HBsAg), HPV, pneumococcal conjugate.
• Toxoid. Inactivated bacterial toxin. Triggers antibodies that neutralise the toxin rather than the bacterium. Examples: tetanus, diphtheria.
• mRNA vaccines. Lipid nanoparticles deliver mRNA encoding a pathogen antigen, which host cells translate into protein for immune presentation. Examples: Pfizer-BioNTech and Moderna COVID-19 vaccines.
• Viral vector vaccines. A harmless virus delivers a gene encoding the pathogen antigen. Examples: AstraZeneca COVID-19 (adenovirus vector).

Herd immunity

When a high proportion of a population is immune to a pathogen, susceptible individuals are indirectly protected because chains of transmission cannot sustain themselves. This is herd immunity (or community immunity).

The threshold depends on how infectious the pathogen is, measured by R0 (the basic reproduction number, the average number of secondary cases produced by one case in a fully susceptible population). The threshold for herd immunity is approximately 1 minus 1/R0.

Reasons herd immunity matters:

• Protects people who cannot be vaccinated for medical reasons (very young infants, immunocompromised patients, transplant recipients).
• Prevents outbreaks even when individual immunity wanes.
• Failure to reach the threshold has driven recent measles outbreaks where vaccination coverage has dropped.

Antibiotic resistance

Antibiotics are drugs that kill bacteria (bactericidal) or stop them growing (bacteriostatic). Different classes have different targets: cell wall synthesis (penicillins, cephalosporins), protein synthesis (tetracyclines, macrolides), DNA replication (quinolones), folate metabolism (sulfonamides).

Antibiotic resistance is the ability of a bacterial strain to grow in the presence of an antibiotic that would normally kill or inhibit it. Resistance evolves rapidly because:

1. Mutation. Random changes in bacterial DNA occasionally produce a resistance gene. With generation times as short as 20 minutes and populations of billions per gram of host tissue, mutations are common.
2. Selection. When antibiotics are present, susceptible bacteria die. Resistant variants survive and reproduce, passing on the resistance gene to their offspring (vertical transmission).
3. Horizontal gene transfer. Plasmids carrying resistance genes can be passed between bacteria of the same or different species by conjugation, transformation or transduction. Multi-drug resistance plasmids spread rapidly.

Mechanisms of resistance.

• Alteration of the antibiotic's target (modified penicillin-binding protein in MRSA).
• Production of enzymes that inactivate the antibiotic (beta-lactamases hydrolyse penicillin).
• Efflux pumps that export the antibiotic out of the cell.
• Reduced permeability of the cell wall and membrane.

Practices that accelerate resistance.

• Patients stopping antibiotic courses early.
• Prescribing antibiotics for viral infections (which they cannot treat).
• Routine use of antibiotics in livestock as growth promoters.
• Over-the-counter sales of antibiotics without medical supervision.
• Poor infection control in hospitals.

Implications for human health.

• "Superbugs" such as methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant Enterococcus (VRE) and multi-drug-resistant tuberculosis (MDR-TB) are major causes of hospital-acquired infection.
• Surgery and chemotherapy depend on effective antibiotics; rising resistance threatens to make routine procedures dangerous again.
• The WHO has identified antimicrobial resistance as one of the top global public health threats. Estimates project that resistance could cause 10 million deaths per year by 2050 if unaddressed.

Slowing resistance.

• Antibiotic stewardship: prescribe only when needed, use the right drug at the right dose, finish the full course.
• Infection control: hand hygiene, isolation of infected patients, vaccination to reduce demand for antibiotics.
• New drug development and alternative therapies (bacteriophage therapy, monoclonal antibodies).
• Reduce livestock use of antibiotics for non-therapeutic purposes.

Common traps

Saying vaccines "fight off the pathogen". The vaccine itself does not fight off anything; it triggers an adaptive response and leaves memory cells.

Confusing antibiotic and antiviral. Antibiotics target bacteria; antivirals target viruses. They are not interchangeable, and prescribing antibiotics for a viral illness drives resistance without helping the patient.

Treating resistance as caused by antibiotics directly. Antibiotics select for pre-existing resistant variants; they do not create resistance through induction. The variation arises by chance through mutation; antibiotics are the selective agent.

Forgetting horizontal gene transfer. Bacteria can swap resistance genes across species. This is why resistance can spread faster than a simple parent-to-offspring model predicts.

Cross-link to Year 12 assessment

This dot point's evolutionary logic foreshadows Unit 4 natural selection (Mendelian variation, selection, allele frequency change) and is a common IA3 research-investigation topic (the rise of resistance, vaccine development, public health strategies). EA Paper 2 extended responses often ask students to evaluate a vaccination programme or interpret resistance data.

In one sentence

Vaccines work by introducing pathogen antigens to trigger a primary adaptive response that leaves memory cells, producing a fast, large secondary response on real exposure; herd immunity protects susceptible people indirectly once coverage exceeds 1 minus 1/R0; and antibiotic resistance evolves through mutation and selection (with horizontal gene transfer accelerating spread), threatening modern medicine unless stewardship, infection control and new drug development slow it.