Investigate and assess the effectiveness of pharmaceuticals as treatment strategies for the control of infectious disease, including: antivirals and antibiotics, the development of antibiotic resistance, and the role of immunisation including the impact of vaccination programs in conferring herd immunity

Top band (5) needs a comprehensive, data-supported judgement. Key points from the guidelines:

• Vaccinations initiate an immune response using a form of the pathogen/antigen that does not cause infection.
• From the graph, the more booster shots, the longer the person stays immune (each dose lifts antibody level above the immune threshold).
• Cases occurring in people over 65 result from not having a booster for a long time (immunity wanes).
• The reduction in tetanus cases shows vaccinations are effective, while the recurring schedule maintains immunity.

Judgement: vaccinations and the booster schedule are effective/valuable. Marker feedback: make an actual judgement (don't just describe the graph) and use all the stimulus.

Source: NESA 2023 HSC Biology examination and marking guidelines.

Full marks (5) require a valid, safe, controlled agar-plate procedure. A strong design includes:

• Inoculate agar plates with a pure culture of a known bacterium (spread evenly).
• Apply discs/wells soaked in honey at one or more concentrations; include a control (e.g. a disc with no honey, or sterile water) for comparison.
• Control variables: same bacterial species/concentration, agar type, plate size, incubation temperature and time.
• Incubate (e.g. 24–48 h) and measure the zone of inhibition (clear area) around each disc; repeat for reliability.
• Safety: work aseptically, sterilise equipment, seal plates and do not reopen them after incubation, wear gloves, disinfect the bench.

Marker feedback: inoculate with a pure culture, compare to a control, and avoid opening dishes after incubation.

Source: NESA 2022 HSC Biology examination and marking guidelines.

Top band (7) needs analysis of the data with arguments for and against the statement. Key points from the guidelines:

• Supports the statement (specific protection): the measles vaccine greatly reduces measles — incidence in vaccinated children is very low/zero vs unvaccinated, and only 2 vaccinated children died of measles vs 40 unvaccinated. Since groups were matched, the vaccine is likely responsible.
• Against the statement (non-specific benefits): vaccinated children also died less from other diseases — about half the rate from diarrhoea/dysentery and under one-third from oedema — suggesting protection beyond the specific disease. But oedema numbers are small, and fever differences are minor, so protection is not uniform.
• Overall: the vaccinated group had about half the total mortality, so the statement requires qualification.

Marker feedback: use all the stimulus and manipulate the data to build arguments both for and against.

Source: NESA 2021 HSC Biology examination and marking guidelines.

3 marks for outlining both a benefit and a limitation. Sample answer: Benefit — antibiotics treat bacterial infections by inhibiting bacterial growth (or killing bacteria). Limitation — antibiotic resistance in bacteria is becoming increasingly common, reducing the effectiveness of many antibiotics. 2 marks for identifying both, or outlining only one. Marker feedback: match the drug to the right pathogen (antibiotics work on bacteria, not viruses); don't confuse antibiotics with antibodies; and note that the bacteria (not the patient's body) develop resistance. Source: NESA 2020 HSC Biology examination and marking guidelines.

1 mark - what antibiotics target. Antibiotics target structures or processes unique to bacteria (for example the peptidoglycan cell wall, the bacterial 70S ribosome or bacterial DNA gyrase).

1 mark - why viruses are unaffected. Viruses lack these structures and use the host cell's machinery to replicate, so there is nothing for an antibiotic to act on without harming the host.

The mark hinges on naming a bacterial-specific target AND stating that viruses use host machinery. Answers that only say "viruses are different" cap at 1 mark.

1 mark - the stimulus

A vaccine introduces a pathogen antigen in a form that does not cause disease (a weakened/inactivated pathogen, a subunit, a toxoid, or mRNA encoding an antigen).

1 mark - the primary response

This triggers an adaptive immune response: B and T lymphocytes specific to the antigen are activated and proliferate.

1 mark - memory

The response generates memory B and T cells, so on later exposure to the real pathogen the response is faster and stronger (secondary response), clearing the pathogen before disease develops.

All three stages (safe antigen, specific response, memory cells) are needed for full marks.

1 mark - the formula

Herd immunity threshold $= 1 - \tfrac{1}{R_0}$.

1 mark - the calculation

$1 - \tfrac{1}{5} = 0.8$, so 80 per cent of the population must be immune.

1 mark - who is protected

Herd immunity protects people who cannot be vaccinated, for example newborns, the severely immunocompromised, or those with severe vaccine allergies (any one).

Showing the substitution into the formula is required; a bare "80%" without working caps at 2 marks.

1 mark - variation

Within a bacterial population there is genetic variation: random mutations mean a few bacteria already carry a resistance allele (e.g. one producing beta-lactamase) before the antibiotic is used.

1 mark - selection pressure

Applying the antibiotic is a selection pressure that kills the susceptible bacteria, while the resistant bacteria survive.

1 mark - differential reproduction

The surviving resistant bacteria reproduce (rapidly, by binary fission), passing the resistance allele to offspring, so its frequency increases in the population.

1 mark - outcome / spread

Over generations the population becomes predominantly resistant; resistance can also spread between bacteria by horizontal gene transfer (conjugation, transformation, transduction).

Band 6 answers stress that the mutation arises randomly and beforehand - the antibiotic selects for it, it does not cause it. Intentional language ("bacteria adapt to survive") loses the natural-selection mark.

Antibiotics (1 mark)

Antibiotics act on bacterial-specific targets (peptidoglycan wall synthesis, the 70S ribosome, DNA gyrase, folate synthesis) that have no human equivalent, so the drug can kill or inhibit the bacterium with little harm to the host.

Antivirals (1 mark)

Antivirals block viral-specific enzymes or life-cycle steps (e.g. reverse transcriptase, protease, neuraminidase, viral polymerase) rather than a free-living cell's structures.

The comparison / difficulty (2 marks)

Because viruses replicate inside host cells using host machinery, there are far fewer virus-only targets, so it is hard to harm the virus without harming the host cell. Antivirals are therefore usually narrowly pathogen-specific, and high mutation rates in RNA viruses make resistance and evasion common, complicating development.

Full marks need a genuine comparison (both drug classes addressed) plus the host-machinery reason for the difficulty.

Mechanisms (3 marks, 1 each).

• Conjugation - a plasmid carrying the resistance gene is transferred from one bacterium to another through a pilus (direct cell-to-cell transfer).
• Transformation - a bacterium takes up free DNA (including resistance genes) released from dead bacteria in its environment.
• Transduction - a bacteriophage carries a resistance gene from one bacterial host to another during infection.

(Also creditable as a vertical mechanism: rapid binary fission passing the gene to daughter cells.)

Strategies (2 marks, 1 each - any two).

• Antibiotic stewardship: prescribe only for confirmed bacterial infection and complete the full course, reducing the selection pressure.
• Reduce agricultural use of antibiotics as growth promoters.
• Hospital infection control (handwashing, isolating resistant cases) and surveillance (e.g. Australia's AURA) to limit transmission.

Each named mechanism must include its vector (plasmid/pilus, free DNA, phage) to earn the mark.

"Assess" requires a judgement weighing effectiveness against limitations for each tool, supported by examples, with an overall conclusion. A Band 6 response is structured and evidenced, not a list.

Antibiotics (1-2 marks)

Highly effective against bacterial infection by targeting bacterial-specific structures (cell wall, 70S ribosome), and cheap and widely available - they have dramatically reduced deaths from bacterial disease. Limitations: ineffective against viruses/fungi/protozoa, kill beneficial gut flora, and their long-term effectiveness is undermined by the evolution of resistance (e.g. MRSA, MDR-TB) through natural selection and horizontal gene transfer.

Antivirals (1-2 marks)

Effective against specific viruses by blocking viral enzymes (e.g. oseltamivir for influenza, reverse-transcriptase inhibitors for HIV), turning some lethal infections into manageable conditions. Limitations: narrowly pathogen-specific, do not eradicate latent virus, and resistance arises readily in high-mutation RNA viruses.

Vaccines (1-2 marks)

The most effective tool for prevention: they pre-arm the adaptive immune system with memory cells, can achieve herd immunity above the $1 - \tfrac{1}{R_0}$ threshold, and have eradicated smallpox (1980) and nearly eliminated polio. Limitations: preventive not curative, require high coverage, some types unsuitable for the immunocompromised, and effectiveness is threatened by vaccine hesitancy.

Judgement (1-2 marks)

A supported conclusion: pharmaceuticals are highly effective overall, with vaccines the most valuable (prevention plus herd immunity) and antibiotics/antivirals essential for treatment, but their effectiveness is conditional on responsible use (stewardship) and high vaccination coverage to counter resistance and waning immunity. An answer lacking an explicit, evidence-based judgement caps below full marks.

Herd immunity threshold (1 mark)

$1 - \tfrac{1}{R_0} = 1 - \tfrac{1}{4} = 0.75$, so 75 per cent of the population must be immune.

Coverage adjustment for efficacy (2 marks)

Because the vaccine is only 80 per cent effective, the proportion vaccinated must exceed the threshold: minimum coverage $= \tfrac{0.75}{0.80} = 0.9375$, i.e. about 94 per cent must be vaccinated for coverage alone to reach the threshold.

Why this is hard (2-3 marks - any of)

• Vaccine hesitancy / misinformation lowers uptake below 94 per cent.
• Access and supply gaps (cost, cold-chain, remote communities) leave pockets unvaccinated, and herd immunity is locality-dependent, so local clusters can still spread.
• Some people cannot be vaccinated (immunocompromised, allergic), and immunity may wane, requiring boosters.
• If R0 rises (a more transmissible variant), the required threshold rises too.

Full marks need both correct calculations (75% threshold, ~94% coverage) shown with working, plus at least two distinct, well-explained barriers.