Biology • Year 12 • Module 7 • Lesson 14
Vaccination — Active and Passive Immunity
Apply the herd immunity threshold formula, interpret real vaccination coverage data, classify immunity types in new scenarios, and critique a student’s diagram.
1. Interpret the graph — measles vaccination coverage and annual case numbers
The graph below plots annual measles cases (left axis, log scale) and vaccination coverage (right axis, %) in a hypothetical country across 15 years. At Year 11, coverage peaked at 97% before falling due to a vaccine hesitancy campaign. 8 marks
Figure 1.1. Vaccination coverage and annual measles cases over 15 years. Adapted from hypothetical national immunisation program data; measles herd immunity threshold ~95%.
1.1 Describe the relationship between vaccination coverage and annual measles cases between Years 1 and 11. What does this suggest about the effectiveness of the vaccination program? 2 marks
1.2 Between Years 11 and 13, coverage dropped from 97% to 89% and cases rose from 3 to 1,240 — a drop of 8 percentage points produced a more than 400-fold increase in cases. Using the concept of herd immunity threshold, explain why a relatively small drop in coverage produced such a dramatic increase in cases. 3 marks
1.3 By Year 15, coverage had recovered to 96% and cases fell to 8. Using the definition of elimination from the lesson, assess whether this country had achieved measles elimination by Year 15. Justify your answer. 3 marks
2. Cause-and-effect chain — what vaccine hesitancy does to a community
Complete the cause-and-effect chain below by filling in each empty effect box. The first cause is provided. The chain traces the consequences of declining measles vaccination rates in an Australian school community. 5 marks (1 per effect + 1 for overall outcome)
Vaccine hesitancy causes measles vaccination rates in a school to drop from 97% to 88%.
Coverage drops below the measles herd immunity threshold of …
3. Classify and justify — real Australian immunity scenarios
For each scenario, identify the type of immunity being described (using all four quadrants of the two-axis framework: natural/artificial × active/passive). Provide a brief justification that references the mechanism. 8 marks (2 per scenario)
3.1 In 2023, a 3-month-old Australian infant was hospitalised with whooping cough (pertussis). The infant had not yet received any vaccinations, but the mother had received a pertussis booster (dTpa) during her third trimester. The infant had some but not full protection. What type of immunity did the infant initially possess, and why was it incomplete and temporary?
3.2 A wildlife veterinarian working in Far North Queensland is bitten by an eastern brown snake (Pseudonaja textilis). She is treated with CSL polyvalent antivenom at the regional hospital. Six months later she receives no ongoing protection against brown snake venom. Name the type of immunity the antivenom provided and explain why protection was not sustained.
3.3 A 45-year-old Australian nurse contracted COVID-19 during the Delta wave in 2021 and recovered. She later received two doses of the Pfizer-BioNTech mRNA vaccine. Describe the type of immunity she developed after each event (infection and vaccination) and explain how the two events together likely contributed to particularly strong immunity.
3.4 Australian health authorities monitor measles vaccination coverage in each state each year, aiming to maintain it above 95%. Explain why maintaining coverage above this figure matters more than simply reaching it once, using the concepts of herd immunity and population vulnerability.
4. Diagram critique — what’s wrong with this student’s immunity classification?
A Year 12 student has drawn the diagram below to classify four immunity scenarios. There are three biological errors. Identify each error and write the correction. 6 marks (2 per error: 1 identify, 1 correct)
4.1 Error 1: What is wrong?
Correction:
4.2 Error 2: What is wrong?
Correction:
4.3 Error 3: What is wrong?
Correction:
Q1.1 — Trend description (2 marks)
Between Years 1 and 11, vaccination coverage rose from 55% to 97% while annual measles cases fell from 8,200 to 3 [1]. The inverse relationship — rising coverage, falling cases — strongly suggests the vaccination program was highly effective at reducing measles transmission [1]. Accept: reference to the cases “approaching near-zero” as coverage crossed 95% (the herd immunity threshold).
Q1.2 — Why a small coverage drop produced a massive case spike (3 marks)
Coverage fell from 97% to 89%, crossing below the measles herd immunity threshold of ~95% [1]. Once coverage falls below the threshold, the pathogen’s effective reproduction number rises back above 1 — each infected person now generates more than one new case on average, so transmission chains can sustain and grow [1]. Near the threshold, the relationship between coverage and case numbers is non-linear: a small decrease in coverage below the threshold re-exposes a disproportionately large number of susceptible individuals to viable transmission chains, allowing the pathogen to spread rapidly through unimmunised pockets of the population [1].
Q1.3 — Assessing elimination at Year 15 (3 marks)
Elimination is defined as the reduction of disease incidence to zero in a defined geographic area, while the pathogen still exists elsewhere [1]. At Year 15, the country recorded 8 cases — not zero [1]. Therefore the country had not achieved elimination by strict definition; it had achieved very low incidence consistent with near-elimination, but 8 cases indicate some residual local transmission or imported cases. A minimum of zero local cases for a sustained period (WHO requires 3 consecutive years for certification) is required for formal elimination status [1].
Q2 — Cause-and-effect chain (sample answers)
Effect 1: Coverage drops below the measles herd immunity threshold of ~95%, breaking the population-level protection that had been preventing transmission chains from sustaining.
Effect 2: Transmission chains re-establish — each infected person can now find susceptible (unvaccinated or incompletely vaccinated) contacts, and the effective R number climbs above 1.
Effect 3: Unvaccinated or incompletely vaccinated students in the school are exposed to circulating measles virus and begin contracting the disease.
Effect 4: Cases spread from the school into the wider community, including to individuals who cannot be vaccinated (e.g. infants under 12 months too young for MMR, immunocompromised individuals) who previously relied on herd immunity for protection.
Overall outcome: A measles outbreak occurs, hospitalising vulnerable individuals who were previously protected by the community’s high vaccination coverage; herd immunity can only be restored by increasing vaccination rates back above 95%.
Q3.1 — Infant and pertussis (2 marks)
Natural passive immunity [1]. The mother’s vaccination during pregnancy boosted her IgG antibody levels, and these maternal IgG antibodies crossed the placenta to the foetus. The infant’s own immune system was not activated; the infant received pre-formed antibodies from the mother without mounting its own response. Protection was incomplete because the concentration of maternal antibodies depends on how recently the mother was vaccinated and how high her titre was — and temporary because the antibodies are catabolised over months; the infant had no memory B cells to sustain immunity [1].
Q3.2 — Eastern brown snake antivenom (2 marks)
Artificial passive immunity [1]. Antivenom consists of pre-formed antibodies produced in another animal (commonly horses); these are injected as a medical intervention to immediately neutralise venom toxins. The veterinarian’s own immune system was not activated to produce antibodies against the venom — no clonal selection of her own lymphocytes occurred, so no memory B or T cells were formed. The antivenom antibodies were catabolised over weeks to months, leaving no lasting protection [1].
Q3.3 — Nurse with COVID-19 infection then vaccination (2 marks)
After infection: natural active immunity — her own immune system mounted a primary response to SARS-CoV-2 antigens, activating B and T cells and producing memory cells. After vaccination: artificial active immunity — the mRNA vaccine introduced spike-protein antigen instructions that triggered her immune system again, producing another round of clonal selection and memory-cell formation [1]. The combination of natural infection followed by vaccination (“hybrid immunity”) typically produces higher, broader, and more durable antibody levels than either alone, because the immune system has been stimulated twice through different antigen presentation contexts, generating a larger and more diverse pool of memory B cells [1].
Q3.4 — Maintaining measles coverage above 95% (2 marks)
Reaching 95% once is insufficient because herd immunity is a dynamic state, not a permanent achievement. Population changes — births add new susceptible individuals, vaccine hesitancy reduces coverage in some cohorts, and immunity wanes or was never fully established in some vaccine non-responders — continuously erode the immune proportion of the population [1]. If coverage falls below 95% even temporarily (as shown in the graph between Years 11 and 13), transmission chains rapidly re-establish and outbreaks occur. Consistent maintenance above the threshold is required because the measles virus still exists globally and importation events occur regularly in Australia; any susceptible cluster provides a foothold for an outbreak [1].
Q4 — Diagram critique (6 marks)
4.1 Error 1 — “Mother’s antibodies cross the placenta” is placed in the Natural Active cell. This is wrong because the infant did not produce its own antibodies; it received pre-formed maternal antibodies without activating its own immune system — this is passive, not active, immunity. Correction: move this example to Natural Passive. Natural Active should contain an example like “contracting and recovering from measles”. [1 + 1]
4.2 Error 2 — “MMR vaccination” is placed in the Artificial Passive cell. This is wrong because vaccination triggers the recipient’s own immune response — clonal selection and memory-cell formation — making it active, not passive. Correction: MMR vaccination belongs in Artificial Active. Artificial Passive should contain examples such as antivenom or immunoglobulin injection. [1 + 1]
4.3 Error 3 — The caption states “Passive immunity produces long-lasting protection because the received antibodies form memory cells.” This is wrong in two ways: passive immunity is temporary (not long-lasting), and the received antibodies do NOT form memory cells because the recipient’s own lymphocytes are not activated. Correction: “Passive immunity provides immediate but temporary protection. The transferred antibodies are gradually broken down (catabolised) over weeks to months, and because the recipient’s own lymphocytes are not activated, no memory B or T cells are formed.” [1 + 1]