Vaccination and Immunity
In 1796, English physician Edward Jenner gave 8-year-old James Phipps the world's first vaccination, a cowpox inoculation that made Phipps immune to smallpox, launching a practice that has since saved an estimated 154 million lives.
Printable Worksheets
Print or save as PDF, or build a custom worksheet from any module's questions.
Q1 · Before you get sick, your body already has ways to defend itself. What do you know about how your immune system remembers diseases it has fought before?
Q2 · A school introduces a vaccination program for a contagious disease. Predict how this might protect students who are not vaccinated as well as those who are.
● Know
- How vaccines stimulate the immune system to produce memory cells
- The difference between active and passive immunity
- What herd immunity is and how it protects communities
● Understand
- Why vaccines do not cause the disease they prevent
- How vaccination schedules are designed
- Why some people cannot be vaccinated and rely on herd immunity
● Can do
- Explain how a vaccine works using the concepts of antigens and memory cells
- Distinguish between active and passive immunity
- Calculate herd immunity thresholds using a simple model
In 1796, 8-year-old James Phipps received a cowpox inoculation from English physician Edward Jenner, and six weeks later, Jenner deliberately exposed the boy to smallpox. Phipps never got sick. That experiment marked the birth of vaccination: a tiny amount of foreign material had taught the immune system to fight a disease it had never fully encountered. Vaccination has since saved more lives than any other medical intervention except clean water.
The answer lies in adaptive immune memory. A vaccine presents your immune system with harmless versions of a pathogen - or just pieces of it - that are enough to trigger an immune response but not enough to cause disease. Your B cells produce antibodies, your T cells activate, and crucially, memory cells form. These memory cells can persist for long periods, although the duration of protection varies between vaccines. When the real pathogen appears, they mount a rapid secondary response that can clear the infection before it makes you seriously ill.
Different vaccines use different strategies. Live attenuated vaccines use weakened viruses that replicate poorly. Inactivated vaccines use killed viruses or bacteria. Subunit vaccines use just a protein from the pathogen. mRNA vaccines give your cells instructions to make the protein themselves. Toxoid vaccines use inactivated toxins. All of these approaches achieve the same goal: training the adaptive immune system without causing disease.
The HPV vaccine contains virus-like particles - empty protein shells that look like human papillomavirus to the immune system but contain no genetic material. Your B cells produce antibodies against these shells, and memory cells form. If real HPV enters your body years later, these antibodies bind to the virus and prevent it from infecting cervical cells, dramatically reducing the risk of cervical cancer.
Australian immunisation success: Australia National Immunisation Program provides free vaccines for 16 diseases, achieving coverage rates above 95% for most childhood vaccines. The program eliminated endemic measles, dramatically reduced meningococcal disease, and is on track to eliminate cervical cancer through HPV vaccination. NCIRS at Westmead Hospital monitors safety and effectiveness.
Vaccines give you the disease. This is false. No modern vaccine contains pathogens capable of causing disease in healthy people. Live attenuated vaccines use weakened viruses that can replicate only slightly - not enough to cause illness in immunocompetent individuals. Inactivated, subunit, and mRNA vaccines cannot replicate at all. Mild fever or soreness after vaccination is a sign that your immune system is responding, not that you have the disease.
A vaccine contains weakened or killed pathogens (or just their proteins). Your immune system responds and creates memory cells. But the vaccine itself does not make you sick. How is this possible?
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.
Immunity comes in two flavours: active and passive. Understanding the difference is essential for understanding vaccines, breast feeding, and antibody therapies.
Active immunity occurs when your own immune system produces antibodies and memory cells. This can happen naturally through infection or artificially through vaccination. Active immunity is often long-lasting because memory cells are formed, but the duration of protection varies between infections and vaccines and some vaccines require booster doses. The downside is that it takes days to weeks to develop.
Passive immunity occurs when you receive ready-made antibodies from another source. Newborns get passive immunity from their mother through the placenta and breast milk. Adults can receive passive immunity through antibody injections, such as tetanus immunoglobulin after a dirty wound or monoclonal antibodies for COVID-19. Passive immunity works immediately but is temporary - the transferred antibodies degrade within weeks to months, and no memory cells are formed.
A traveller heading to a region with hepatitis A might receive the hepatitis A vaccine (active immunity) several weeks before departure, giving their body time to develop memory cells. If they are exposed unexpectedly before the vaccine has taken effect, they might receive an injection of hepatitis A immunoglobulin (passive immunity) for immediate short-term protection.
Australian maternal health: Australian breastfeeding guidelines recommend exclusive breastfeeding for the first six months because breast milk contains antibodies (especially IgA) that provide passive immune protection against respiratory and gastrointestinal infections. This passive immunity bridges the gap until the infant own adaptive immune system matures.
Passive immunity is just as good as active immunity. This is false. Passive immunity is a temporary rental of antibodies, not ownership. It provides no memory, so once the antibodies degrade, you are vulnerable again. Active immunity is like learning to swim; passive immunity is like wearing a life jacket. The life jacket helps immediately, but you still cannot swim on your own when it is removed.
- Active immunity
- Passive immunity
- Vaccination
- Breast milk antibodies
- Provides passive immunity to infants
- Body produces its own antibodies and memory cells
- Ready-made antibodies transferred from another source
- Induces active immunity without causing disease
Herd immunity is one of the most important concepts in public health. It occurs when enough people in a community are immune to a disease that chains of transmission become less likely. This can indirectly protect people who are not immune by reducing their chance of being exposed.
The herd immunity threshold depends on how contagious the disease is. A simple idealised model estimates the threshold as 1 - 1/R0. For example, if R0 were 2, the model gives about 50% immune. Real-world thresholds also depend on factors such as vaccine effectiveness, changes in transmission and how people mix. Because measles is extremely contagious, public-health programs aim for at least 95% coverage with two measles-containing vaccine doses to protect communities and maintain elimination.
Herd immunity is especially important for protecting people who cannot be vaccinated: newborn babies, people with compromised immune systems, and those with severe allergies to vaccine components. These individuals benefit from the immunity of those around them.
During a measles outbreak, a community with 90% vaccination coverage can still have enough susceptible people for transmission to continue. At very high coverage, such as around 95% or more with two doses, transmission becomes much less likely and people who cannot be vaccinated gain indirect protection. High coverage reduces risk; it does not guarantee that every outbreak stops immediately.
Australian herd immunity: Australia maintains high childhood vaccination coverage through the National Immunisation Program and family payments linked to immunisation status. However, some communities have lower coverage due to vaccine hesitancy. Outbreaks of measles and whooping cough in these communities demonstrate how quickly herd immunity can weaken when coverage falls.
If everyone else is vaccinated, I do not need to be. This is false. Herd immunity is a collective effect that depends on high coverage across the whole community. If too many people are susceptible, outbreaks become more likely. Furthermore, no vaccine is 100% effective; even vaccinated people benefit from lower transmission because it reduces their chance of exposure.
Vaccine safety is one of the most studied topics in medical science. Before any vaccine is approved for public use, it undergoes years of rigorous testing. Preclinical testing in cells and animals establishes basic safety and immune response. Phase 1 trials test safety in a small group of healthy volunteers. Phase 2 trials test immune response and dosing in hundreds of people. Phase 3 trials compare the vaccine against a placebo in thousands or tens of thousands of people to prove efficacy and monitor rare side effects. Even after approval, post-marketing surveillance continues to detect extremely rare adverse events.
The claim that vaccines cause autism originated from a fraudulent 1998 study by Andrew Wakefield, who was later found to have falsified data and had undisclosed financial conflicts of interest. The study was retracted, Wakefield lost his medical license, and over 20 subsequent studies involving millions of children have found no link between vaccines and autism.
Vaccines do contain small amounts of chemicals like aluminium salts (to boost immune response) and formaldehyde (to inactivate viruses). But the quantities are minuscule compared to everyday exposure. Your body naturally produces more formaldehyde than is in any vaccine, and breast milk contains more aluminium than vaccines.
The COVID-19 vaccines were developed rapidly, but they did not skip safety steps. They used pre-existing mRNA technology platforms, received massive funding, and enrolled tens of thousands of volunteers simultaneously. Phase 3 trials for Pfizer and Moderna each involved over 30,000 participants. Post-marketing surveillance through systems like VAERS in the US and AusVaxSafety in Australia has monitored hundreds of millions of doses, confirming that serious adverse events are extremely rare.
Australian vaccine safety: AusVaxSafety is a national surveillance system that monitors vaccine safety in real time. When Australians receive vaccines at participating clinics, they receive a follow-up survey about any side effects. This system detected that the AstraZeneca COVID-19 vaccine was associated with a very rare blood clotting disorder, enabling rapid regulatory response and informed consent. This demonstrates that vaccine safety monitoring is continuous and responsive.
Vaccines are not tested enough. This is false. Vaccines undergo more rigorous testing than most prescription medications. They are tested in healthy children and adults, pregnant women, elderly people, and immunocompromised patients. The safety database for common childhood vaccines includes hundreds of millions of doses administered over decades. No medical intervention is completely risk-free, but vaccines have one of the most favourable risk-benefit ratios of any treatment.
Wrong: "Vaccines contain the live disease and can give you the illness." Not true for most vaccines. While live attenuated vaccines contain weakened pathogens, they are designed to be unable to cause disease in people with healthy immune systems. Inactivated, subunit, and mRNA vaccines cannot cause disease at all.
Right: Most vaccines use inactivated, subunit, or mRNA components that cannot cause disease. Live attenuated vaccines contain weakened pathogens specifically designed to be unable to cause illness in immunocompetent individuals.
Wrong: "If everyone else is vaccinated, I do not need to be." This is dangerous thinking. Herd immunity only works when most people are vaccinated. If too many people refuse vaccination, herd immunity breaks down and outbreaks occur, putting vulnerable people at risk.
Right: Herd immunity requires high vaccination coverage across the whole community to be effective. Relying on others' immunity while refusing vaccination yourself puts vulnerable people who cannot be vaccinated at risk and can lead to outbreaks.
Wrong: "Vaccines are not natural, so they must be harmful." Vaccines work by using the body's natural immune system. They simply provide antigens in a safe way, allowing the body to produce its own natural immunity without the risks of actual disease.
Right: Vaccines harness the body's natural immune system by presenting antigens safely, enabling the body to produce its own immunity and immune memory without the dangers of actual infection.
Australian Vaccination Successes
The National Immunisation Program (NIP): Australia's free vaccination program provides immunisation against 17 diseases for children, adolescents, and adults. Diseases that were once common, measles, polio, diphtheria, tetanus, whooping cough, are now rare thanks to high vaccination rates. The NIP includes vaccines given at birth, 2 months, 4 months, 6 months, 12 months, 18 months, 4 years, and throughout adolescence.
Measles elimination: Australia was declared measles-free in 2014 by the World Health Organization, meaning the disease no longer circulates continuously. However, imported cases still occur when unvaccinated travellers bring measles from overseas. Maintaining at least 95% two-dose vaccination coverage is the public-health target used to protect communities and prevent re-establishment.
HPV vaccination: Australia was the first country to introduce a national HPV vaccination program (2007) and is on track to become the first country to eliminate cervical cancer. The vaccine protects against human papillomavirus, which causes cervical cancer, throat cancer, and genital warts. By vaccinating both girls and boys, Australia is protecting the entire population.
✍ Copy Into Your Books
▾How Vaccines Work
- Present antigens without causing disease
- Trigger primary immune response
- Produce memory B and T cells
- Real infection = rapid secondary response
Types of Immunity
- Active: body makes its own antibodies and memory cells
- Passive: receives ready-made antibodies
- Active = often long-lasting; duration varies; Passive = temporary
Herd Immunity
- High vaccination rate reduces transmission across a community
- Protects those who cannot be vaccinated
- Threshold depends on disease contagiousness and real-world factors
Vaccine Science
Evaluate a Myth
At the start of this lesson, you were asked how the measles vaccine can shield unvaccinated babies, elderly people, and cancer patients who never received the shot themselves, the puzzle of herd immunity.
Now that you've studied vaccination and immunity, can you explain exactly how vaccinating enough people in a community protects those who can't be vaccinated? How has your thinking about the purpose of vaccines changed?
Q1. 1. Describe how vaccination stimulates immunity without causing disease. Use the concepts of antigens, B cells, and memory cells in your answer. 4 MARKS
Q2. 2. Explain the difference between active and passive immunity, including one example of each and why active immunity is usually longer-lasting. 4 MARKS
Q3. 3. Using the idealised model threshold ≈ 1 - 1/R0, estimate the herd immunity threshold for a new infectious disease with R0 = 4, then explain why high population immunity is important for public health. 4 MARKS
Revisit Your Thinking
Go back to your Think First answer. Has your understanding changed?
- Can you now explain how a vaccine trains your immune system without making you sick?
- Why is herd immunity important for protecting people who cannot be vaccinated?
Model answers (click to reveal)
Answers
▾MCQ 1
B Vaccines present antigens to the immune system, triggering a primary response and producing memory cells without causing the actual disease.
MCQ 2
B Passive immunity involves receiving ready-made antibodies. Maternal antibodies crossing the placenta provide temporary protection to the newborn.
MCQ 3
B Herd immunity occurs when a high proportion of the population is immune, reducing disease spread and indirectly protecting those who cannot be vaccinated for medical reasons.
MCQ 4
B Live attenuated vaccines contain weakened pathogens that have been grown in conditions that reduce their ability to cause disease, while still stimulating an immune response.
MCQ 5
C Measles is extremely contagious and public-health programs aim for at least 95% coverage with two measles-containing vaccine doses to maintain community protection.
Short Answer 1
Model answer: Vaccination stimulates immunity by presenting pathogen antigens to the immune system in a safe form. The vaccine contains weakened, killed, or partial pathogens (or genetic instructions to make pathogen proteins) that cannot cause disease. When these antigens are detected, B cells recognise them and become activated. Activated B cells multiply and differentiate into plasma cells, which produce antibodies specific to the vaccine antigens. Some B cells become memory B cells that can persist for long periods. If the vaccinated person later encounters the real pathogen, these memory cells can trigger a rapid secondary immune response, producing large amounts of antibodies that help neutralise the pathogen.
Short Answer 2
Model answer: Active immunity occurs when the body's own immune system produces antibodies and memory cells in response to an antigen. Examples include vaccination (e.g., measles vaccine) or natural infection. Active immunity is often longer-lasting because memory B and T cells are formed, although the duration of protection varies and some vaccines require booster doses. Passive immunity occurs when a person receives ready-made antibodies from another source. Examples include maternal antibodies crossing the placenta during pregnancy and antivenom injections for snake bites. Passive immunity is temporary because the transferred antibodies break down over time and no memory cells are produced.
Short Answer 3
Model answer: Using the idealised formula threshold ≈ 1 - 1/R0, a disease with R0 = 4 gives 1 - 1/4 = 0.75, or approximately 75%. In this simple model, that level of immunity is associated with preventing sustained transmission. High population immunity is important because it reduces transmission and indirectly protects vulnerable people who cannot be vaccinated. In real populations, the actual level needed can differ because vaccine effectiveness, transmission patterns and population mixing also matter.