Modes of Transmission

Q1 — Sample Band 6 response (8 marks), annotated

Epidemic curve. The epidemic curve shows two discrete peaks: the first on Day 2–3 (22 new cases) and the second on Day 4–5 (19 new cases), with zero new cases in the intervals between and after. This is a two-peak point-source pattern, not a propagated pattern, because each peak is sharp and discrete rather than a progressive wave. [1 — curve described correctly as two-peak point source]

Cluster 1 — transmission mode. The first cluster (22 cases, Days 2–3) was caused by indirect contact via contaminated food: the chicken pasta served at Day 1 dinner. Evidence: all 22 cases ate that meal; onset was 24–48 hours later, consistent with the Campylobacter incubation period (24–72 h); the chef failed to wash hands after raw poultry, likely contaminating the food. [1 — mode (indirect food-borne) with evidence]

Cluster 2 — transmission mode. The second cluster (19 cases, Days 4–5) was caused by direct contact: the unaffected dormitory room-mates of Cluster 1 cases were exposed through close contact with symptomatic individuals (faecal-oral route via shared bathroom facilities, contaminated surfaces in the room). Evidence: all 19 new cases had shared rooms with Cluster 1 cases; they became ill approximately 48 hours after sustained close contact, consistent with C. jejuni transmission via faecal-oral contact. [1 — mode (direct contact / faecal-oral) with evidence]

Why Cluster 2 is not a propagated pattern. A propagated outbreak produces successive waves of increasing size as each generation of cases infects the next, separated by one incubation period each time. At Camp Kosciuszko there were only two discrete peaks and the outbreak then ceased entirely — new cases did not continue to generate further waves. Once Cluster 2 cases were separated into a different cabin on Day 5, transmission stopped, consistent with a point-source pattern for each cluster rather than self-sustaining person-to-person spread. [1 — correctly distinguishes two-peak point source from propagated]

Snow’s method applied. Applying John Snow’s approach: the epidemiologists would map cases (who got sick, when, where they slept and ate), identify the common sources (Day 1 dinner for Cluster 1; shared dormitory rooms for Cluster 2), and intervene by removing the source. The Day 5 action of moving symptomatic students to a separate cabin mirrors Snow’s pump-handle removal: it targeted the transmission route (direct contact via shared living space) directly and stopped new cases. This intervention correctly addressed direct-contact transmission — rather than, say, treating the food supply after it was already consumed. [1 — Snow’s method correctly applied; intervention evaluated as correctly targeted]

Judgement on control strategy. A single control measure would not have prevented both clusters because the two clusters had different transmission routes. Preventing Cluster 1 required food-safety intervention at the source: hand hygiene for kitchen staff and safe handling of raw poultry. Preventing Cluster 2 required early isolation of symptomatic individuals to break direct-contact transmission before Cluster 2 cases accumulated. A combination of food safety protocols (targeting indirect contact) and rapid case isolation (targeting direct contact) would have been necessary. Relying on isolation alone would have done nothing to prevent the food-borne Cluster 1; relying on food safety alone would not have stopped dormitory-based direct contact in Cluster 2 once Cluster 1 cases were symptomatic. [1 — evidence-based judgement requiring combination of controls; 1 — explicit link between control choice and transmission route; 1 — uses lesson terms (point source, indirect contact via food, direct contact, isolation) with precision]

Marking criteria (8 marks).

• 1 mark — Epidemic curve correctly described as two-peak point source (not propagated), with the discrete timing of each peak identified.
• 1 mark — Cluster 1 correctly identified as indirect contact (food-borne) with at least one piece of supporting evidence from the data.
• 1 mark — Cluster 2 correctly identified as direct contact (faecal-oral/close contact) with at least one piece of supporting evidence from the data.
• 1 mark — Correct explanation of why two discrete peaks do not constitute a propagated pattern (no successive self-sustaining waves; outbreak ended).
• 1 mark — Snow’s epidemiological method (case mapping → source identification → targeted intervention) correctly described and applied to the outbreak.
• 1 mark — The Day 5 intervention (separating symptomatic students) evaluated as correctly targeting the direct-contact route for Cluster 2.
• 1 mark — Judgement reaches an explicit conclusion that a combination of controls (food safety + isolation) was needed because the two clusters had different transmission routes.
• 1 mark — Response uses precise lesson terminology throughout (point source, indirect contact, direct contact, epidemic curve, incubation period, transmission route).

Q2 — Sample Band 6 response (7 marks), annotated

Overall judgement. The claim is partially defensible but contains a critical biological error that makes it incorrect as a universal public health principle. [1 — clear evaluative opening]

What is defensible. Isolating infected individuals is an effective and appropriate control measure for diseases that spread primarily via direct contact between people. For influenza (transmitted by respiratory droplets >5 µm when an infected person coughs or sneezes in close proximity), removing the infectious person from shared spaces directly interrupts the transmission chain — there is no alternative environmental reservoir that will continue to spread the virus. Similarly, isolating COVID-19 patients reduces droplet and aerosol transmission to close contacts. For these diseases, the claim is broadly correct. [1 — defensible element correctly identified with named example and mechanism]

Critical biological error. The claim is false for diseases transmitted by indirect contact or by vectors, because in those cases the infected person is not the direct source of transmission to the next host — the intermediate (contaminated water, fomite, or vector organism) is. For cholera (Vibrio cholerae, waterborne), isolating a patient does not remove the bacteria already contaminating the water supply. People can continue to be infected by drinking from the same water source even after every known patient has been isolated. The correct control is water treatment and sanitation — targeting the intermediate, not the person. For malaria and dengue (vector-borne), the virus/parasite circulates in mosquito populations; isolating a patient does not kill mosquitoes or prevent them from biting other people. Mosquito control (insecticides, draining breeding sites, bed nets) must target the vector. [1 — error identified; 1 — cholera example correctly used; 1 — vector-borne example correctly used]

Deeper flaw. The claim also overlooks John Snow’s foundational lesson: he halted the 1854 Broad Street cholera outbreak not by isolating patients but by removing the pump handle — targeting the contaminated water source. Even before anyone knew the pathogen, Snow understood that the transmission route determined the correct intervention. Misidentifying the mode leads to ineffective interventions: isolating cholera patients while leaving the water supply contaminated fails, just as draining mosquito ponds is irrelevant to controlling influenza. [1 — Snow’s principle correctly applied as counter-evidence]

Defensible reformulation. “The most effective control measure for an infectious disease must target the specific transmission route by which the pathogen reaches new hosts. For diseases transmitted by direct contact between people (e.g. influenza, COVID-19), isolating infected individuals breaks the chain. For waterborne diseases (e.g. cholera), the primary intervention must target the contaminated water source. For vector-borne diseases (e.g. malaria, dengue), the primary intervention must control the vector organism. A control strategy that misidentifies the transmission route will be ineffective regardless of how rigorously it is applied.” [1 — biologically defensible reformulation that links transmission mode to control choice]

Marking criteria (7 marks).

• 1 mark — States an overall evaluative judgement (e.g. “partially defensible but contains a critical error”).
• 1 mark — Correctly identifies the defensible element: isolation works for direct-contact diseases (influenza or COVID-19 named and mechanism stated).
• 1 mark — Correctly identifies the critical error: isolation does not work when the transmission route is indirect or vector-mediated.
• 1 mark — Uses cholera as a named example and correctly explains why isolating the patient does not remove the contaminated water source.
• 1 mark — Uses a vector-borne disease (malaria or dengue) as a named example and correctly explains why isolating the patient does not control the vector.
• 1 mark — Applies John Snow’s principle (or equivalent reasoning) that the intervention must target the transmission route, not just the infected person.
• 1 mark — Produces a biologically defensible reformulation that explicitly links mode of transmission to the appropriate control strategy, without claiming one strategy is universally superior.