Year 9 Science · Unit 1 · Lesson 10
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Learning Goals
Read the graph
Study the bar chart showing herd immunity thresholds (HIT) for six diseases, then answer the questions below.
Herd Immunity Thresholds for Selected Infectious Diseases
Data: WHO Immunization Programme; CDC; NCIRS Australia (2024)
(a) Which disease shown in the graph has the highest herd immunity threshold, and what does this tell you about how contagious it is?
(b) The formula for herd immunity threshold is: HIT = 1 − 1/R₀. Measles has an R₀ of 15. Show your working to calculate the HIT for measles, and check whether it matches the bar in the graph.
(c) Flu has a much lower threshold than measles. Why is it still hard to eliminate influenza from the community, even though the threshold looks achievable?
Real-world context
In a regional town of 10,000 people, 6,500 residents are currently vaccinated against a disease with R₀ = 4. A traveller arrives in the town carrying the disease. Town health officials are trying to decide whether an outbreak is likely.
(a) Using the formula HIT = 1 − 1/R₀, calculate the herd immunity threshold for this disease. Show your working.
(b) What percentage of the town is currently vaccinated? Has the town reached the herd immunity threshold? Explain your reasoning.
(c) Predict what will happen when the infected traveller arrives. Who in the community is most at risk? Justify your answer using the concept of herd immunity.
1. Explain the "free rider problem" in herd immunity. Why is it a risk to the whole community, not just to the individual who chooses not to vaccinate?
2. The 2015 Disneyland measles outbreak in California began in a theme park and spread to 147 people across several states. Using your knowledge of herd immunity, suggest what conditions allowed this outbreak to occur.
Wrap Up
In one sentence, what was the main idea of this lesson?