Year 9 Science · Unit 1 · Lesson 10
Challenge Worksheet
Learning Goals
Evaluate the claim
Someone claims...
"Herd immunity can be achieved naturally through infection rather than vaccination, this is a perfectly valid alternative public health strategy. If enough people simply catch the disease and recover, the community will be just as well protected as it would be through mass vaccination, and without the risks of injecting everyone with a vaccine. Sweden tried a natural approach to COVID-19 and the herd immunity still built up eventually."
(a) What part of this claim is supported by the science you have learned? Explain which scientific ideas the claim gets right.
(b) What is misleading or missing from this claim? Identify at least two major problems with "natural herd immunity" as a public health strategy. Think about cost, time, who gets harmed, and how long immunity lasts.
(c) What evidence or extra information would help you decide whether the claim about Sweden is a valid comparison? What would you need to know about their outcomes?
(d) The 2019 Samoa measles epidemic killed 83 people, mostly children under 5, after vaccination coverage fell to 31%. Use the herd immunity threshold formula (HIT = 1 − 1/R₀) and your knowledge of natural immunity to explain why relying on natural infection was catastrophic in this case. Include specific numbers in your answer.
1. A new vaccine prevents a disease with R₀ = 8, but the vaccine is only 85% effective (meaning some vaccinated people are still slightly susceptible). Explain whether vaccinating 90% of the population would realistically achieve herd immunity. Show any calculations you use.
2. A scientist states: "Herd immunity is not a goal, it is a fragile side effect of high vaccination rates, and it can vanish almost overnight." Do you agree with this statement? Justify your answer using evidence from real outbreaks and your understanding of the threshold concept.
Wrap Up
In one sentence, what was the main idea of this lesson?