Herd Immunity and Vaccination Coverage
During a 2019 measles outbreak in Sydney, NSW Health traced every case back to classrooms where vaccination coverage had dropped below 90%, demonstrating that protecting a population does not require 100% immunity, just enough.
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Q1 · How can a disease stop spreading in a community even when not every person is vaccinated?
Q2 · A school has 95% vaccination coverage for measles. Predict what might happen if coverage dropped to 80% after a social media campaign discouraging vaccines.
● Know
- Define herd immunity and the herd immunity threshold
● Understand
- Explain how a disease's R0 sets the vaccination coverage needed to protect a population
● Can do
- Evaluate how high vaccination coverage shields vulnerable people, using measles and influenza as examples
In 2020, the world watched public health authorities report two very different numbers for COVID-19: an early case fatality rate above 3% in Italy, and under 0.5% in South Korea, the same disease, seemingly different danger. The difference came from how many cases were being detected, not how deadly the virus was. Three key statistics let epidemiologists make sense of exactly this kind of data. Case fatality rate (CFR) tells us how deadly a disease is: it is the percentage of people diagnosed with a disease who die from it. Vaccine efficacy tells us how well a vaccine works in trials. Incidence rate tells us how fast a disease is spreading in a population, adjusted for population size.
Always check the context behind the numbers. A raw case count of 50 might represent a crisis in a village of 500 but a blip in a city of 5 million. A case fatality rate of 5% might be terrifying for Ebola but actually quite low for rabies. Numbers are tools, not truths, they only become meaningful when you know what they measure, who they include, and what they leave out.
A disease outbreak causes 50 deaths out of 1,000 confirmed cases. The case fatality rate is (50 / 1000) x 100 = 5%. This means 5% of people diagnosed with this disease die from it. It does not mean 5% of the entire population will die.
The Australian Bureau of Statistics and AIHW publish disease statistics using standardised rates per 100,000 population, allowing fair comparison between states and between Australia and other countries.
A depth study is your opportunity to think and work like a real scientist. The process begins with curiosity: what disease-related question genuinely interests you? From there, you narrow it down to an investigable question one that is specific enough to answer, testable through measurement or observation, and linked to the scientific concepts you have studied in this unit.
Next, you develop a hypothesis, design a fair method with clear variables, collect reliable data, analyse it with appropriate graphs and statistics, and draw conclusions that honestly evaluate whether your evidence supports your hypothesis. The best depth studies do not just confirm what the textbook says; they explore something new, even if the result is unexpected. Science advances through unexpected results.
A student interested in hygiene might start with Does handwashing matter? and refine it to Does 20 seconds of soap handwashing reduce bacterial colony count more than 5 seconds? The refined version is specific (time-based), testable (agar plate counts), and linked to pathogen transmission concepts.
The Science Teachers Association of NSW runs the Young Scientist Awards, celebrating depth studies from students statewide. Winners often go on to represent Australia at international science fairs.
Herd immunity is the protection of a whole community when enough people are immune to a disease that it cannot spread easily. The threshold, the percentage of people who need to be immune, depends on how contagious the disease is.
The formula: Herd immunity threshold ≈ 1 − 1/R₀
Where R₀ (the basic reproduction number) is the average number of people one infected person infects in a fully susceptible population.
Examples:
- Measles: R₀ ≈ 15 → threshold ≈ 1 − 1/15 = 0.933, or about 93%.
- Polio: R₀ ≈ 5 → threshold ≈ 1 − 1/5 = 0.80, or about 80%.
- Influenza: R₀ ≈ 2 → threshold ≈ 1 − 1/2 = 0.50, or about 50%.
If vaccination coverage falls below the threshold, outbreaks can occur even in highly vaccinated communities, because clusters of unvaccinated people create pockets where the disease can spread.
In 2019, Samoa experienced a devastating measles outbreak that killed 83 people, mostly children. The outbreak occurred because vaccination coverage had fallen to about 31% after a tragic incident in 2018 where two infants died from a vaccine preparation error (nurses mixed the vaccine with anaesthetic instead of saline). This caused widespread vaccine hesitancy, and coverage plummeted. When measles was introduced, it spread rapidly through the unvaccinated population. The outbreak ended only after an emergency vaccination campaign reached over 90% coverage. This tragedy illustrates how fragile herd immunity is, a small drop below the threshold can have catastrophic consequences, especially for highly contagious diseases like measles.
Australian vaccination coverage: The National Centre for Immunisation Research and Surveillance (NCIRS) monitors childhood vaccination coverage across Australia. National coverage for the full schedule at age 5 is about 94%, just above the measles threshold. However, pockets of lower coverage exist, particularly in some affluent areas where vaccine hesitancy is higher. In 2019, several measles outbreaks occurred in Australia linked to imported cases in under-vaccinated communities. The Australian Immunisation Register tracks every vaccination, enabling public health officials to identify areas at risk and target interventions. Maintaining >95% coverage for measles-mumps-rubella (MMR) vaccine is essential for keeping measles eliminated in Australia.
Measles has an R0 of about 15, requiring ~95% herd immunity threshold. In a community where vaccination coverage drops to 85%, predict what will happen when a single measles case is introduced.
How close was your prediction?
Nice calibration, your intuition is good for this kind of problem.
Good, being surprised is the point. This answer is worth remembering.
Wrong: "A depth study is just a long essay about a disease." No, a depth study is an investigation. It requires you to ask a question, gather evidence, analyse data, and draw conclusions. It is active science, not just research.
Right: A depth study is an active scientific investigation that requires asking a question, gathering evidence, analysing data, and drawing conclusions, it is not simply a research essay.
Wrong: "The different topics in this unit have no connection to each other." No, they are deeply connected. Pathogens cause disease, which the immune system fights, which vaccines train, which antibiotics treat, which resistance limits, which public health prevents. Every topic links to others.
Right: Every topic in this unit is connected, pathogens cause disease, the immune system fights infection, vaccines train immunity, antibiotics treat bacteria, and resistance drives public health strategies.
Wrong: "Once you memorise facts about disease, you understand it." No, true understanding means being able to explain connections, apply concepts to new situations, and evaluate evidence. Facts are tools; understanding is the ability to use them.
Right: True understanding means explaining connections between concepts, applying knowledge to new situations, and evaluating evidence, not just recalling isolated facts.
Australian Scientists Fighting Disease
Professor Fiona Stanley (AC): An Australian epidemiologist who founded the Telethon Kids Institute in Perth. Her research on birth defects, Indigenous health, and population health methods transformed Australian public health. She championed the use of population data to guide health policy.
Professor Ian Frazer: Co-developer of the HPV vaccine at the University of Queensland. His work has prevented countless cases of cervical cancer worldwide and put Australia on track to eliminate cervical cancer entirely.
Modern Australian research: Today, Australian scientists at WEHI, the Doherty Institute, CSIRO, and universities across the country continue to fight disease. During COVID-19, Australian researchers contributed to vaccine development, genomic surveillance, and long COVID research. Aboriginal and Torres Strait Islander researchers are increasingly leading health research that addresses community priorities with cultural authority.
✍ Copy Into Your Books
▾Unit Connections
- Pathogen -> Transmission -> Defence -> Treatment
- Infectious vs non-infectious disease
- Local, national, and global perspectives
Key Formulas
- Herd immunity threshold ≈ 1 - 1/R0
- Incidence rate = (new cases/population) × multiplier
- Case fatality rate = (deaths/cases) × 100%
Depth Study Steps
- Choose topic -> Formulate question -> Research -> Hypothesis -> Method -> Data collection -> Analysis -> Conclusions -> Communication
Concept Connections
Depth Study Planning
At the start of this lesson, you considered the 2019 measles outbreak in a Sydney school, where cases erupted only in classrooms where vaccination coverage had slipped below 90%, even though none of the students had been sick before.
Now that you've worked through the lesson, can you explain exactly why a disease disappears when enough people are immune, even if some are not? Has your view of your own role in community health changed?
Q1. Define herd immunity and explain what is meant by the herd immunity threshold. 2 marks
Q2. Influenza has an R0 of about 2, while measles has an R0 of about 15. Using the formula threshold ≈ 1 minus 1/R0, calculate the herd immunity threshold for each and explain why measles needs much higher vaccination coverage. 3 marks
Q3. A baby too young to be vaccinated lives in a community with 95% vaccination coverage. Explain how the baby is protected, and evaluate what could happen to the baby if coverage in the community fell well below the threshold. 3 marks
Revisit Your Thinking
Go back to your Think First answer. Has your understanding changed?
- How has your understanding of disease and health developed across this entire unit?
- What connections between concepts do you find most powerful or surprising?
Model answers (click to reveal)
Answers
▾MCQ 1
C Herd immunity is the protection of a whole community that happens when enough people are immune that the disease cannot spread easily. It does not require every person to be vaccinated.
MCQ 2
A Using 1 minus 1/R0 with R0 = 15 gives 1 minus 1/15 = 0.933, or about 93%. Highly contagious diseases like measles need very high coverage.
MCQ 3
D When coverage is high, there are very few susceptible people left, so the disease cannot find a chain of hosts to reach those who cannot be vaccinated. This indirect protection is the whole point of herd immunity.
MCQ 4
B The basic reproduction number (R0) is the average number of people one infected person infects in a fully susceptible population. A higher R0 means a more contagious disease and a higher herd immunity threshold.
MCQ 5
A The measles threshold is about 93%, so dropping to 80% leaves coverage below the threshold. Enough susceptible students now exist for an introduced measles case to start an outbreak.
Short Answer 1
Model answer: Herd immunity is the protection of a whole community that occurs when enough people are immune to a disease (through vaccination or past infection) that the disease cannot spread easily from person to person. The herd immunity threshold is the percentage of the population that must be immune to stop the disease spreading. Once coverage is at or above this threshold, even people who are not immune are indirectly protected because the disease cannot reach them.
Short Answer 2
Model answer: For influenza, threshold ≈ 1 minus 1/2 = 0.5, or about 50%. For measles, threshold ≈ 1 minus 1/15 = 0.933, or about 93%. Measles needs much higher vaccination coverage because its R0 is far larger, meaning each infected person spreads the disease to many more people. With so many secondary infections per case, a much larger fraction of the population must be immune to break the chains of transmission and stop the disease spreading.
Short Answer 3
Model answer: The baby is protected by herd immunity. Because 95% of the community is immune, there are very few susceptible people, so a disease such as measles cannot spread and is very unlikely to ever reach the baby, even though the baby has no immunity of its own. If coverage fell well below the threshold (for example to 70%), there would be enough susceptible people for the disease to spread through the community. The chains of transmission could then reach the unvaccinated baby, who is at high risk of serious illness because newborns have immature immune systems. This shows that the baby relies entirely on others staying vaccinated.