Training the body’s defences
Vaccination is one of the most important medical inventions in history. It has saved hundreds of millions of lives by preventing diseases like smallpox, polio, measles and diphtheria. The World Health Organization estimates that vaccines save millions of lives every year.
How the immune system works
Your immune system defends against germs like bacteria and viruses.
- When a new germ enters, white blood cells recognize it as an invader.
- The body makes antibodies, proteins that latch onto that specific germ.
- Fighting a new germ the first time takes days, so you may get sick.
- Afterward, memory cells remember the germ.
- If it returns, your body fights it off quickly, often before you feel sick. This is immunity.
How vaccines work
A vaccine safely teaches your immune system to recognize a germ without making you sick with the disease. It contains something that looks like the germ to your body.
| Vaccine type | What it contains | Examples |
|---|---|---|
| Live weakened | A weakened form of the germ | Measles, mumps, rubella (MMR), chickenpox |
| Inactivated | A killed germ | Polio (injected), most flu shots |
| Subunit or protein | Just a piece of the germ | Hepatitis B, HPV, whooping cough |
| Toxoid | An inactivated toxin the germ makes | Tetanus, diphtheria |
| mRNA | Instructions for your cells to make one harmless germ protein | Some COVID-19 vaccines |
| Viral vector | A harmless virus carrying instructions | Some Ebola and COVID-19 vaccines |
Some vaccines need several doses or boosters to build strong, lasting protection.
History of vaccination
| Year | Event |
|---|---|
| 1000s–1700s | Variolation: in China, India, Africa and the Ottoman Empire, people were deliberately infected with mild smallpox to gain protection |
| 1796 | Edward Jenner (England) uses cowpox to protect a boy, James Phipps, from smallpox: the first vaccine (from Latin vacca, cow) |
| 1885 | Louis Pasteur (France) develops a rabies vaccine |
| 1914 | Connaught Laboratories founded in Toronto to make diphtheria antitoxin |
| 1920s | Vaccines for diphtheria, tetanus and whooping cough |
| 1955 | Jonas Salk’s polio vaccine; Connaught Labs in Toronto produced much of the vaccine for the large U.S. field trial |
| 1960s | Measles, mumps and rubella vaccines |
| 1980 | Smallpox declared eradicated |
| 2006 | HPV vaccine prevents several cancers |
| 2020 | First mRNA vaccines, for COVID-19, developed in under a year |
Canadian contributions include work on the Ebola vaccine at the National Microbiology Laboratory in Winnipeg, which was approved in 2019.
Successes
| Disease | Achievement |
|---|---|
| Smallpox | Killed an estimated 300 million people in the 20th century; eradication declared in 1980, the first human disease wiped out |
| Polio | Cases fell over 99% since 1988; eliminated from the Americas in 1994 |
| Measles | Vaccination saved an estimated tens of millions of lives since 2000 |
| Diphtheria | Once a leading killer of Canadian children; now very rare |
Herd immunity
When most people in a community are vaccinated, a germ has trouble spreading. This herd immunity protects people who can’t be vaccinated, like newborn babies or people with weakened immune systems.
How many people need to be immune depends on how contagious the disease is. For measles, about 95% of people need to be immune.
Safety
Vaccines are tested in large clinical trials before approval and are continuously monitored afterward by Health Canada and the Public Health Agency of Canada.
- Common side effects are mild: a sore arm, mild fever or tiredness for a day or two.
- Serious side effects are very rare.
- Large studies involving millions of children have found no link between vaccines and autism. A 1998 study claiming a link was found to be fraudulent and was retracted.
Challenges
- Vaccine hesitancy: misinformation has led some people to skip vaccines, allowing diseases like measles to return in some places.
- Access: some parts of the world lack vaccines, cold storage or health workers.
- Changing germs: flu viruses change each year, so new flu vaccines are needed annually.
Common mistakes
- Thinking vaccines give you the disease. Most can’t cause the disease at all.
- Thinking natural infection is safer. Diseases like measles can cause brain damage or death.
- Thinking diseases are gone, so vaccines aren’t needed. Diseases can return if vaccination drops.
Try it yourself
- Make a timeline of five vaccines and the diseases they prevent.
- Simulate herd immunity: in a grid of 100 squares, mark 90 as "vaccinated." Start an "infection" in one square that spreads to unvaccinated neighbours. Try again with 50 vaccinated.
- Ask a family member to look at your vaccination record. Which diseases are you protected against?
Quick recap
- Vaccines train the immune system to recognize germs without causing disease.
- Edward Jenner created the first vaccine in 1796.
- Vaccines eradicated smallpox and nearly eliminated polio.
- Herd immunity protects people who can’t be vaccinated.
- Vaccines are carefully tested and monitored for safety.