Vaccination and Immunity
Vaccines are one of humanity's greatest achievements. They have eradicated smallpox, nearly eliminated polio, and saved more lives than any other medical intervention. A simple injection trains your immune system to recognise and destroy deadly pathogens — without you ever getting sick.
Before You Begin
Think about vaccines you may have received: for measles, tetanus, HPV, or COVID-19.
Write down your answers before reading on:
- What do you think a vaccine actually does inside your body?
- Why do some vaccines need multiple doses?
- Why do we vaccinate people who are not at high risk of serious illness?
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
How Vaccines Work
Training the immune system without the disease
B Cells
Vaccines work by presenting antigens to the immune system without causing the actual disease. This tricks the body into mounting a primary immune response and producing memory cells.
There are several types of vaccines:
- Live attenuated: Weakened live pathogen that cannot cause disease in healthy people (e.g., measles, mumps, rubella vaccine)
- Inactivated: Killed pathogen that cannot replicate (e.g., polio vaccine, hepatitis A vaccine)
- Subunit/recombinant: Only a piece of the pathogen (a protein or sugar) is used (e.g., hepatitis B vaccine, HPV vaccine)
- mRNA: Contains genetic instructions for making a pathogen protein (e.g., Pfizer and Moderna COVID-19 vaccines)
- Toxoid: Inactivated toxin that trains the body to neutralise the real toxin (e.g., tetanus, diphtheria vaccines)
After vaccination, if the real pathogen is encountered, memory cells trigger a rapid secondary response that destroys it before disease develops.
Active vs Passive Immunity
Two ways to become immune
Immunity can be acquired in two fundamentally different ways:
Active immunity occurs when the body produces its own antibodies and memory cells. This happens through:
- Natural infection — catching a disease and recovering
- Vaccination — deliberate exposure to antigens
Active immunity is long-lasting (often lifelong) because memory cells persist for years or decades.
Passive immunity occurs when ready-made antibodies are transferred to a person. This happens through:
- Maternal antibodies crossing the placenta during pregnancy
- Antibodies in breast milk (colostrum)
- Injection of antibodies (e.g., antivenom for snake bites, monoclonal antibodies for COVID-19)
Passive immunity is temporary (weeks to months) because no memory cells are produced. The transferred antibodies eventually break down.
Herd Immunity
Protecting the community
Herd immunity occurs when a high proportion of a population is immune to a disease, making it difficult for the disease to spread. This protects vulnerable individuals who cannot be vaccinated, such as:
- Newborns too young for certain vaccines
- People with weakened immune systems (e.g., cancer patients)
- People with severe allergies to vaccine components
- People for whom a particular vaccine is ineffective
The herd immunity threshold depends on how contagious the disease is:
| Disease | Basic reproduction number (R0) | Herd immunity threshold |
|---|---|---|
| Measles | 12-18 | ~95% |
| Polio | 5-7 | ~85% |
| COVID-19 (original) | 2.5-3 | ~70% |
| Influenza | 1-2 | ~50% |
Highly contagious diseases like measles require very high vaccination rates to achieve herd immunity. Even small drops in vaccination coverage can lead to outbreaks.
Vaccine Safety and Myths
Separating fact from fiction
Vaccines are among the most thoroughly tested and monitored medical interventions in history. Before approval, they undergo:
- Preclinical testing: Laboratory and animal studies
- Phase 1 trials: Safety testing in small groups
- Phase 2 trials: Testing in larger groups for effectiveness and side effects
- Phase 3 trials: Large-scale testing involving thousands of participants
- Post-marketing surveillance: Ongoing monitoring for rare side effects
Common myths debunked:
Myth: "Vaccines cause autism." Fact: This claim originated from a fraudulent 1998 study that was retracted. Dozens of large studies involving millions of children have found no link between vaccines and autism.
Myth: "Vaccines contain dangerous toxins." Fact: Vaccines contain ingredients in amounts far too small to cause harm. For example, the aluminium in vaccines is less than what you consume in food daily.
Myth: "Natural immunity is better than vaccine immunity." Fact: While natural infection often produces strong immunity, it comes with the risk of severe disease, complications, and death. Vaccines provide protection without these risks.
Common Misconceptions
"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.
"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.
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 high vaccination coverage (>95%) is essential to 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 = long-lasting; Passive = temporary
Herd Immunity
- High vaccination rate protects entire community
- Protects those who cannot be vaccinated
- Threshold depends on disease contagiousness