B. Biology: Tissues, Organs, and Systems of Living Things · Tissues, Organs, and Systems of Living ThingsPublic Health Strategies: Vaccines, Screening and Education
B1.3 A1.8 A1.9 A1.12 A1.13 A2.1 A2.2About 70 minutes
Learning goals- I can describe public health strategies related to systems biology, such as vaccination, cancer screening and HIV/AIDS education.
- I can explain how vaccines work with the immune system and how community immunity protects people who can't be vaccinated.
- I can use data to assess the impact of a public health strategy, including calculating a community immunity threshold.
- I can describe public health careers and the work of Canadian public health leaders such as Dr. Sheela Basrur.
Words to know
public health
Work that protects and improves the health of whole populations, mainly by preventing disease.
vaccine
A preparation that trains the immune system to recognize a germ without causing the disease.
antibody
A protein made by white blood cells that attaches to a specific germ and helps destroy it.
memory cells
White blood cells that "remember" a germ so the body can respond quickly next time.
community (herd) immunity
When enough people are immune that a disease can't spread easily, protecting those who aren't immune.
R₀ (basic reproduction number)
The average number of people one infected person infects in a population with no immunity.
screening
Testing people who have no symptoms to find a disease early.
false positive / false negative
A test result that wrongly says a person has the condition / wrongly says they don't.
The doctor for a whole city
A family doctor treats one patient at a time. A public health team looks after everyone at once. They check that tap water is safe, inspect restaurants, run vaccination clinics in schools, track outbreaks, and run campaigns about smoking, sun safety and sexual health. When public health works, nothing happens: no outbreak, no poisoned water. That's why it is easy to forget how much it matters. In this lesson you'll look at public health strategies linked to the body systems you've studied, and learn to judge their impact with data.
How vaccines work
Your immune system (part of your circulatory and lymphatic systems) defends against germs:
- A vaccine contains a harmless form or piece of a germ (a weakened or inactivated virus, a protein, or instructions for making one).
- White blood cells recognize it as foreign. Some make antibodies that fit that germ exactly.
- The body keeps memory cells. If the real germ arrives later, the memory cells respond so fast that the person usually doesn't get sick, or gets much less sick.
Examples in the curriculum:
- MMR vaccine protects against measles, mumps and rubella. Measles is one of the most contagious diseases known and can cause pneumonia and brain swelling. Ontario's Immunization of School Pupils Act requires students to have certain vaccines, including MMR, or a valid exemption.
- HPV vaccine protects against human papillomavirus types that cause most cervical cancers and some other cancers. Ontario offers it free to students in Grade 7. It works best when given before any exposure to the virus, which is why it is offered at that age.
The first vaccine (1796) used cowpox to protect against smallpox. After a worldwide vaccination campaign, the World Health Organization declared smallpox eradicated in 1980: the only human disease wiped out so far.
Community immunity
Some people can't be vaccinated: newborn babies, people having cancer treatment, people with certain immune conditions. They are protected when enough people around them are immune, because the germ can't find enough new hosts to keep spreading.
How many people need to be immune? It depends on how contagious the disease is, measured by R₀: the average number of people one sick person infects when no one is immune. A simple model gives:
immunity threshold = 1 − 1/R₀
Worked exampleLet's calculate a community immunity threshold
Disease X has R₀ = 5. (a) What share of the population must be immune to stop it spreading? (b) The vaccine protects 90% of people who get it. What share of the population must be vaccinated?
- (a) Threshold = 1 − 1/R₀ = 1 − 1/5 = 1 − 0.2 = 0.80, so 80% must be immune.
- (b) Only 90% of vaccinated people become immune, so: share vaccinated × 0.90 = 0.80.
- Share vaccinated = 0.80 ÷ 0.90 ≈ 0.889 → about 89%.
- Interpretation: because no vaccine is perfect, the vaccination target must be higher than the immunity threshold.
(a) 80% immune; (b) about 89% of the population must be vaccinated.
Cancer screening
Screening means testing healthy people with no symptoms to find a disease early, when it is easier to treat (remember how tumours grow by doubling). Ontario runs organized screening programs for breast cancer (mammograms, a type of X-ray), cervical cancer (testing for HPV or abnormal cells), colorectal cancer (a home stool test) and lung cancer (low-dose CT for people at high risk, such as long-time smokers). Each program invites people in certain age groups and sends reminders.
Screening has trade-offs:
- False positives cause worry and extra tests for people who are actually healthy.
- False negatives can miss a cancer, giving false reassurance.
- Some cancers found may have grown so slowly they would never have caused harm.
Public health experts compare the lives saved with these harms and costs when they decide who should be screened.
Worked exampleLet's interpret screening data
In a screening program, 1000 people are tested. 60 get a positive result. Follow-up tests show that only 6 of those 60 actually have cancer. One more person with a negative result is later found to have cancer. Analyse these results.
- True positives: 6. False positives: 60 − 6 = 54.
- Share of positive results that were real: 6 ÷ 60 = 0.10 = 10%. So most people with a positive result did not have cancer, which is why a positive screening test leads to further testing, not a diagnosis.
- False negatives: 1. People with cancer in total: 6 + 1 = 7. The test found 6 ÷ 7 ≈ 86% of them.
- Assessment: the program found 6 cancers early (a big benefit for those people) but caused worry and follow-up tests for 54 healthy people. Decisions about screening weigh both.
54 false positives, 1 false negative; only 10% of positives were real cancer, but the test caught about 86% of the cancers present.
HIV/AIDS education
HIV (human immunodeficiency virus) attacks a type of white blood cell (helper T cells) that coordinates the immune system. Without treatment, the immune system weakens over years until the body can't fight infections it would normally handle; this late stage is AIDS.
- HIV is spread through certain body fluids: blood, sexual fluids and breast milk. It is not spread by hugging, sharing food, toilets, mosquitoes or casual contact.
- Prevention strategies taught in public health education include condom use, never sharing needles, testing, and medicines that prevent infection in people at higher risk.
- Treatment with antiretroviral drugs lets people with HIV live long, healthy lives. A person whose treatment keeps the virus at an undetectable level does not pass HIV to sexual partners.
- Education campaigns since the 1980s have reduced new infections and fought stigma, which matters because fear of discrimination can stop people from getting tested.
Measuring impact
To assess a public health strategy, look for evidence: case numbers before and after, vaccination or screening rates, death rates, costs, and who is being reached (and who is being missed). Public health works best when it is designed with communities, including First Nations, Inuit and Métis communities and their own health organizations, so that programs fit local needs and earn trust.
Canadian public health leader: Dr. Sheela Basrur
Dr. Sheela Basrur was Toronto's Medical Officer of Health during the SARS outbreak in 2003. She became the public face of the response, giving clear, calm daily updates, and helped control the outbreak through contact tracing and quarantine. She later became Ontario's Chief Medical Officer of Health and pushed to strengthen the province's public health system. She also championed smoke-free public places.
Careers in public health
- Epidemiologist: studies patterns and causes of disease in populations, using statistics (usually a master's degree).
- Public health nurse: runs vaccination clinics, sexual health and school programs (nursing degree).
- Public health inspector: inspects restaurants, water and pools to prevent illness (a degree in environmental health and certification).
- Health promoter: designs education campaigns (university or college programs in health studies).
- Medical Officer of Health: a physician who leads a region's public health unit (medical school and specialty training).
Just for funFunny but true: the word vaccine comes from the Latin word vacca, meaning cow. The first vaccine, in 1796, used material from cowpox sores to protect people from smallpox. So every vaccination clinic has a little bit of cow in its history.
Hands-on activityGlitter germs and the data of handwashing
You need: hand lotion, glitter or ground cinnamon (as pretend germs), a timer, soap and water, paper towel, and a family member or two. Choose cinnamon if you'd rather avoid glitter mess.
- Rub a little lotion on one person's hands, then sprinkle on the "germs." That person shakes hands with someone, opens a door, and touches a pencil. Track where the "germs" end up.
- Test handwashing: have three people try to remove the germs: (a) rinse with cold water for 5 seconds; (b) soap and water for 10 seconds; (c) soap and water for 20 seconds, scrubbing between fingers and under nails. Rate how much is left on a scale of 0 to 5 and record it in a table.
- Graph the results. What does the evidence say about the best method?
- Bonus: with a grown-up, look at your own immunization record (Ontario families can access it through their public health unit). Which vaccines did you get, and which diseases do they prevent?
What to look for: "germs" spread to many surfaces from one person, just as real microbes do; 20 seconds with soap and scrubbing removes much more than a quick rinse. This simple strategy is a major part of public health education. Clean surfaces afterward.
Your turn
1Explain how a vaccine protects you from a disease.
It exposes the immune system to a harmless form or part of the germ. White blood cells make antibodies and memory cells. If the real germ appears, memory cells respond quickly, preventing or reducing illness.
2What three diseases does the MMR vaccine prevent?
Measles, mumps and rubella.
3Why is the HPV vaccine offered in Grade 7?
It works best before any exposure to the virus; giving it early provides protection well ahead of time. It prevents infections that can cause cervical and other cancers.
4Who benefits from community immunity, and why?
People who can't be vaccinated or don't respond to vaccines (babies, people with weak immune systems). If most people around them are immune, the germ can't spread to reach them.
5Calculate the community immunity threshold for a disease with R₀ = 3.
6What is the purpose of cancer screening, and what is one drawback?
To find cancer early in people with no symptoms, when it is easier to treat. Drawbacks: false positives (worry, extra tests), false negatives, cost.
7Name two ways HIV is not spread.
Any two: hugging, shaking hands, sharing food or drinks, toilet seats, mosquito bites, coughing or sneezing.
More practice: warm-up
The average number of people one infected person infects when no one is immune.
2Which disease was declared eradicated in 1980?
3Which type of white blood cell does HIV attack?
4What kind of imaging does breast cancer screening use?
Mammography (low-dose X-ray).
5Who was Toronto's Medical Officer of Health during SARS in 2003?
More practice: on your own
1Disease Y has R₀ = 10. What share must be immune? If the vaccine is 95% effective, what share must be vaccinated?
Threshold = 1 − 1/10 = 90%. Vaccinated share = 0.90 ÷ 0.95 ≈ 0.947 → about 95%.
2In a school of 800 students, 92% are vaccinated against measles. How many are not? Is the school above the 95% target?
8% of 800 = 64 students not vaccinated. No, 92% is below the 95% target, so an outbreak could spread.
3A screening test gave 40 positive results; 4 people really had the disease. What percentage of positive results were false positives?
False positives = 40 − 4 = 36; 36 ÷ 40 = 90%.
4Explain why stigma can make an HIV epidemic worse.
Fear of judgment or discrimination stops people from getting tested, learning their status and starting treatment, so more people unknowingly pass on the virus.
5Choose one public health strategy and give two kinds of data you would collect to assess its impact.
Example: school HPV vaccination. Data: vaccination rates by year and region; rates of HPV infections and cervical pre-cancers in young adults before and after the program (also cost per case prevented, and who is not being reached).
Challenge
1Two diseases have R₀ = 1.5 and R₀ = 15. Compare their immunity thresholds and explain what this means for vaccination programs.
R₀ = 1.5: 1 − 1/1.5 ≈ 33%. R₀ = 15: 1 − 1/15 ≈ 93%. A highly contagious disease needs almost everyone immune, so even small drops in vaccination (for example, from 95% to 90%) can let outbreaks happen, while the less contagious disease can be stopped with much lower coverage.
2A city's measles vaccination rate fell from 95% to 85%. The city's population is 600 000. How many more people are unvaccinated, and why are public health officials concerned?
10% of 600 000 = 60 000 more unvaccinated people. With measles's high R₀, 85% is well below the ~93 to 95% needed, so the virus could spread through clusters of unvaccinated people, also endangering babies and people who can't be vaccinated.
3A new screening test finds 99% of cancers but has many false positives. Another finds 80% with few false positives. Which would you choose for a rare cancer screened in a million people? Justify your answer.
Consider the trade-off. For a rare cancer, even a small false-positive rate in a million people means thousands of healthy people sent for invasive follow-up tests; many positives would be false. A reasonable answer: use the more specific test (fewer false positives) for mass screening, possibly with the sensitive test for high-risk groups, or use the sensitive test first and confirm positives with the second. Strong answers weigh harms of missed cancers against harms of false alarms.
Quiz
2. A disease has R₀ = 4. What is the community immunity threshold?
3. Which is a cancer screening strategy?
5. What is a false positive?
6. Which disease was declared eradicated by the WHO in 1980?
Big idea: expectation B1.3 asks students to describe public health strategies related to systems biology (cancer screening and prevention, HPV and MMR vaccines, AIDS education) and assess their impact on society. The math (thresholds, percentages, false positives) builds data-analysis skills.
Sensitive topics: HIV is taught here factually, in line with the curriculum; Grade 10 Health covers sexual health in more detail. Be ready for questions, and keep the focus on facts and respect. Families have different views on vaccines; the lesson presents the scientific evidence and Ontario's requirements.
How to help: Look at your teen's immunization record together. Do the glitter or cinnamon handwashing test. Talk about a recent public health message you've seen and ask: what is the evidence it works?
Common difficulties:
- Thinking a positive screening test means a person definitely has the disease.
- Forgetting that imperfect vaccines need higher coverage than the immunity threshold.
- Myths about how HIV spreads.