Chapter 4 of 8

All 8 chapters
  1. 1Kinds of disease, readunderstood
  2. 2How diseases spread, readunderstood
  3. 3The body's defences, readunderstood
  4. 4Vaccination, readunderstood
  5. 5Antibiotics and medicines, readunderstood
  6. 6Malaria, HIV and tuberculosis, readunderstood
  7. 7Lifestyle and disease, readunderstood
  8. 8Sexual health, readunderstood

S4.4.4 Chapter 4

Vaccination

How can a vaccine protect me from a disease I have never had?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

  • You picked D. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Explain how a vaccine trains the body to respond faster
  • Explain how herd immunity protects people who cannot be vaccinated

In the last chapter, memory cells gave a fast response the second time a pathogen got in. The first time, the response was slow, and the person may have been ill. A vaccine gives the body that first meeting safely. This is vaccination.

Draw it first

How a vaccine works

  1. Boxes joined by arrows, from top to bottom. The strip grows by one box in each picture. The boxes read: A vaccine has a harmless form of the pathogen; White blood cells make antibodies and memory cells; The real pathogen arrives later; Memory cells give a fast, big response.A vaccine has a harmless formof the pathogenWhite blood cells makeantibodies and memory cells

    1 The vaccine holds a harmless form of the pathogen. White blood cells meet its antigens, make antibodies and leave memory cells.

  2. Boxes joined by arrows, from top to bottom. The strip grows by one box in each picture. The boxes read: A vaccine has a harmless form of the pathogen; White blood cells make antibodies and memory cells; The real pathogen arrives later; Memory cells give a fast, big response.A vaccine has a harmless formof the pathogenWhite blood cells makeantibodies and memory cellsThe real pathogen arriveslater

    2 Days or weeks later, the real pathogen gets into the body.

  3. Boxes joined by arrows, from top to bottom. The strip grows by one box in each picture. The boxes read: A vaccine has a harmless form of the pathogen; White blood cells make antibodies and memory cells; The real pathogen arrives later; Memory cells give a fast, big response.A vaccine has a harmless formof the pathogenWhite blood cells makeantibodies and memory cellsThe real pathogen arriveslaterMemory cells give a fast, bigresponse

    3 The memory cells make the right antibodies quickly and in large amounts. The pathogen is destroyed before it can make the person ill.

What is in a vaccine? It holds the antigens of a pathogen, but it cannot cause the full disease. There are several ways to do this. The pathogen may be dead. It may be weakened. The vaccine may hold only a part of the pathogen, such as a protein from its surface. Some newer vaccines give the body's cells instructions to make just one antigen.

Because of this, a vaccine is given to prevent a disease. It is not a cure for someone who is already ill. It prepares the body before the real pathogen comes.

A line graph. The x axis is days, from 0 to 40 in steps of 10. The y axis is antibody level, from 0 to 100 in steps of 20. The points are at 0 days and 0, 10 days and 10, 20 days and 30, 30 days and 20, 32 days and 60, 35 days and 90, 40 days and 70. The line rises slowly after the vaccine, which is marked vaccine, and fast after the real pathogen arrives, which is marked pathogen. Under the graph two lines read: Vaccine given on day 0. Pathogen arrives on day 30. The antibody level is in made up units.010203040020406080100vaccinepathogendaysantibody levelVaccine given on day 0.Pathogen arrives on day 30.
Antibody level after a vaccine on day 0 and the real pathogen on day 30. The rise after the vaccine is slow and small. The rise after the pathogen is fast and large. Antibody level is in made up units. Example numbers.

Vaccines are tested for safety before they are used, in the way that you will see in the next chapter. Mild effects are common, such as a sore arm or a short fever. They are signs that the body is responding. A side effect is an effect of a medicine that is not the one wanted. Serious side effects are rare. Health workers keep watching and recording them after a vaccine is in use.

No vaccine works on every person every time. Some people cannot be vaccinated. Babies may be too young. Some people have an illness or a treatment that weakens their white blood cells. For them, a vaccine would not work, or would not be safe. How are they protected?

A pathogen needs a path from person to person. Each person who is immune is a break in the path. If enough people are immune, the pathogen reaches few people. The people who cannot be vaccinated are then protected by the rest. This is herd immunity.

We can show it with a model. People stand in a grid. A person who is infected passes the disease to each neighbour who is not vaccinated. A neighbour is the person next to them, above, below, left or right. Vaccinated people are hatched. A cross marks an infected person.

A grid of 4 rows and 4 columns of people, 16 in all. 0 people are vaccinated and drawn hatched. People who are infected carry a cross. The key names three kinds of person: vaccinated, infected and not vaccinated. The caption says 16 of 16 people infected.vaccinatedinfectednot vaccinated16 of 16 people infected
Nobody is vaccinated. The infection starts with one person and spreads to everybody.
A grid of 4 rows and 4 columns of people, 16 in all. 8 people are vaccinated and drawn hatched. People who are infected carry a cross. The key names three kinds of person: vaccinated, infected and not vaccinated. The caption says 6 of 16 people infected.vaccinatedinfectednot vaccinated6 of 16 people infected
Half of the people are vaccinated. The infection reaches only some of the people who are not. Two of them stay well.
A grid of 4 rows and 4 columns of people, 16 in all. 12 people are vaccinated and drawn hatched. People who are infected carry a cross. The key names three kinds of person: vaccinated, infected and not vaccinated. The caption says 1 of 16 people infected.vaccinatedinfectednot vaccinated1 of 16 people infected
Three quarters of the people are vaccinated. Only the first person is infected. The others who are not vaccinated are protected by the people around them.

See it three ways

As a picture

A table with the columns Vaccinated, Infected. 0 of 16, 16 of 16. 8 of 16, 6 of 16. 12 of 16, 1 of 16.VaccinatedInfected0 of 1616 of 168 of 166 of 1612 of 161 of 16

In words

The more people are vaccinated, the fewer people are infected, and the people who cannot be vaccinated are safer. The grids are a model. Real people meet in many ways, so real numbers differ.

How many people need to be immune? It depends on the disease. The more infectious a disease is, the larger the share must be. Measles is highly infectious, so about 19 people in every 20 need to be immune.

Worked example

Use the grid where three quarters of the people are vaccinated. What percentage of the 16 people are vaccinated?

  1. The grid has 4 rows of 4 people, so 4 × 4 = 16 people.
  2. Vaccinated people: 12.
  3. Percentage = 12 ÷ 16 × 100 = 75.

Answer: 75%

Worked example

Use the grid where half of the people are vaccinated. Six people are infected. How many people who are not vaccinated stayed well?

  1. People not vaccinated: 16 − 8 = 8.
  2. People infected: 6, all of them not vaccinated.
  3. Not vaccinated and still well: 8 − 6 = 2.

Answer: 2 people

Try it

Spread an infection on paper

You need: a sheet of paper and a pencil, a cup of dry beans: 12 of one kind and 8 of another, a ruler.

Be safe: Keep beans out of mouths, and away from small children. Do not eat the beans you used.

  1. Draw a grid of 4 rows and 4 columns of squares on the paper. That is 16 squares, each one a person.
  2. Use 12 of the first kind and 4 of the second. Shake these 16 beans in the cup. Tip them out and put one bean on each square. The first kind stands for a vaccinated person.
  3. Pick any square with a bean of the other kind. That person is infected. Mark it with a cross.
  4. Mark a cross on each square next to it, above, below, left or right, that holds a bean of the other kind. Keep going from every new cross until no more squares can be marked.
  5. Count the crosses. Then repeat with 8 of each kind, and compare.

What you will see: With 12 vaccinated people, the infection usually reaches only a few. With only 8, it usually reaches more. Your result changes with how the beans fall, so repeat it a few times. The picture is one example.

If you cannot try it, this is what happens.
A table with the columns Vaccinated, Crosses. 12 of 16, a few. 8 of 16, more.VaccinatedCrosses12 of 16a few8 of 16more

A wrong idea many people have

A vaccine gives you the disease it protects against.

Why it feels right: A vaccine does hold a form of the pathogen, and some people have a sore arm or a short fever afterwards.

The test that breaks it: Many vaccines hold only dead pathogens or a part of one, which cannot multiply or cause the disease. Where a weakened pathogen is used, it is weakened so that it cannot cause the full disease in a healthy person.

What is true: A vaccine trains the body without causing the disease. A mild effect such as a sore arm shows that the white blood cells are responding.

Key words

vaccine
a harmless form of a pathogen, or a part of one, given to prepare the body.
vaccination
giving a vaccine, so that the body can make memory cells before the real pathogen comes.
side effect
an effect of a medicine that is not the one wanted.
immune
able to destroy a pathogen quickly, because memory cells can make the right antibody at once.
herd immunity
protection of people who are not immune, because enough of the others are immune that the pathogen cannot spread.

Back to your guess

The answer is D: A vaccine shows the body a harmless form of the pathogen, so it learns to defend itself before the real one comes.

A vaccine trains the body. It holds a dead or weakened pathogen, or a part of one. White blood cells make antibodies against it and leave memory cells. When the real pathogen comes, the response is fast and big.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. What does a vaccine hold?

    Tap your answer.

    1. Yes, that is right. The antigens in a vaccine let white blood cells make antibodies and memory cells, but the vaccine cannot cause the full disease.

    2. Not this one. A vaccine does not hold antibodies. It trains your own white blood cells to make them. Try another.

    3. Not this one. A vaccine does not kill a pathogen that is already in the body. It prepares the body before the pathogen comes. Try another.

    4. Not this one. That would cause the disease. A vaccine holds a dead or weakened pathogen, or only a part of one. Try another.

    See the answer

    A harmless form of a pathogen, or a part of one.

    The antigens in a vaccine let white blood cells make antibodies and memory cells, but the vaccine cannot cause the full disease.

    • If you chose B: A vaccine does not hold antibodies. It trains your own white blood cells to make them.
    • If you chose C: A vaccine does not kill a pathogen that is already in the body. It prepares the body before the pathogen comes.
    • If you chose D: That would cause the disease. A vaccine holds a dead or weakened pathogen, or only a part of one.

  2. Why is a person vaccinated against a disease often safe from it later?

    Tap your answer.

    1. Not this one. The vaccine does not kill pathogens. It leaves memory cells in the blood. Try another.

    2. Not this one. The skin does not change. The protection comes from white blood cells. Try another.

    3. Yes, that is right. Vaccination leaves memory cells in the blood. If the real pathogen arrives, they respond fast, before it can make the person ill.

    See the answer

    Memory cells make the right antibody quickly and in large amounts.

    Vaccination leaves memory cells in the blood. If the real pathogen arrives, they respond fast, before it can make the person ill.

    • If you chose A: The vaccine does not kill pathogens. It leaves memory cells in the blood.
    • If you chose B: The skin does not change. The protection comes from white blood cells.

  3. What is herd immunity?

    Tap your answer.

    1. Not this one. The word herd is used because it is about a group. It applies to people too. Try another.

    2. Not this one. Herd immunity does not need anyone to be ill. Vaccination can make enough people immune. Try another.

    3. Yes, that is right. Each immune person is a break in the path of the pathogen. When there are enough breaks, those who are not immune are protected too.

    4. Not this one. Herd immunity is not about antibodies passing between people. It works because the pathogen finds too few people to infect. Try another.

    See the answer

    When enough people are immune that the pathogen cannot spread far, so people who are not immune are protected too.

    Each immune person is a break in the path of the pathogen. When there are enough breaks, those who are not immune are protected too.

    • If you chose A: The word herd is used because it is about a group. It applies to people too.
    • If you chose B: Herd immunity does not need anyone to be ill. Vaccination can make enough people immune.
    • If you chose D: Herd immunity is not about antibodies passing between people. It works because the pathogen finds too few people to infect.

  4. Look at the grid. How many people are infected when the infection stops spreading?

    A grid of 4 rows and 4 columns of people, 16 in all. 4 people are vaccinated and drawn hatched. People who are infected carry a cross. The key names three kinds of person: vaccinated, infected and not vaccinated. The four people in the third column are vaccinated. The infected person is in the first column, second row. Everyone else is not vaccinated.vaccinatedinfectednot vaccinated
    See the answer

    8 people

    The infection starts in the left column and spreads only to unvaccinated neighbours. It fills the two left columns, which is 4 + 4 = 8 people. It cannot pass the vaccinated column, so the people on the right stay well.

  5. Look back at the grid where three quarters of the 16 people are vaccinated. How many of the 16 people are not infected?

    See the answer

    15 people

    The picture shows 1 person infected out of 16 people. 16 − 1 = 15.

  6. A grid has a column of vaccinated people between the first infected person and the last column. Why are the people in the last column not infected, although they are not vaccinated?

    Tap your answer.

    1. Not this one. Distance alone does not protect them. The infection reaches anyone it can pass to, one neighbour at a time. Try another.

    2. Yes, that is right. The vaccinated column is a break in every path to the last column. That is how herd immunity protects people who are not vaccinated.

    3. Not this one. People who are not vaccinated can be infected, as the first two columns show. These four are protected only because of the vaccinated column. Try another.

    See the answer

    Their neighbours on the other side are vaccinated, so the infection cannot reach them.

    The vaccinated column is a break in every path to the last column. That is how herd immunity protects people who are not vaccinated.

    • If you chose A: Distance alone does not protect them. The infection reaches anyone it can pass to, one neighbour at a time.
    • If you chose C: People who are not vaccinated can be infected, as the first two columns show. These four are protected only because of the vaccinated column.

  7. Which people does herd immunity help most?

    Tap your answer.

    1. Yes, that is right. These people have no memory cells of their own. When most of the people around them are immune, a pathogen rarely reaches them.

    2. Not this one. It does not need every person. A large share of immune people is enough to stop most spread, and the share depends on the disease. Try another.

    3. Not this one. The vaccinated people have their own protection. Herd immunity is the extra protection for those who are not immune. Try another.

    See the answer

    People who cannot be vaccinated, such as babies who are too young.

    These people have no memory cells of their own. When most of the people around them are immune, a pathogen rarely reaches them.

    • If you chose B: It does not need every person. A large share of immune people is enough to stop most spread, and the share depends on the disease.
    • If you chose C: The vaccinated people have their own protection. Herd immunity is the extra protection for those who are not immune.

  8. Which statement about vaccines is true?

    Tap your answer.

    1. Yes, that is right. Vaccines are tested before they are used and checked afterwards. Mild effects show the body responding, and serious effects are rare.

    2. Not this one. Every vaccine is tested for safety, and health workers keep watching after it is in use. Try another.

    3. Not this one. A vaccine prepares the body before the pathogen comes. It is not a cure. Try another.

    4. Not this one. A vaccine cannot cause the full disease. A mild effect is the body responding. Try another.

    See the answer

    They are tested for safety. Mild effects such as a sore arm are common, and serious effects are rare.

    Vaccines are tested before they are used and checked afterwards. Mild effects show the body responding, and serious effects are rare.

    • If you chose B: Every vaccine is tested for safety, and health workers keep watching after it is in use.
    • If you chose C: A vaccine prepares the body before the pathogen comes. It is not a cure.
    • If you chose D: A vaccine cannot cause the full disease. A mild effect is the body responding.

  9. From an earlier chapter

    What is an antigen?

    Tap your answer.

    1. Not this one. That is a phagocyte. An antigen is a chemical on the surface of a pathogen. Go to S4.4.3. Try another.

    2. Yes, that is right. White blood cells notice an antigen as foreign and make an antibody that fits it.

    3. Not this one. That is an antibody. An antigen is on the pathogen, and the antibody fits it. Go to S4.4.3. Try another.

    See the answer

    A chemical on the surface of a pathogen that the body does not make.

    White blood cells notice an antigen as foreign and make an antibody that fits it.

    • If you chose A: That is a phagocyte. An antigen is a chemical on the surface of a pathogen. Go to S4.4.3.
    • If you chose C: That is an antibody. An antigen is on the pathogen, and the antibody fits it. Go to S4.4.3.

  10. In a town of 200 people, 150 are vaccinated. What percentage of the people are vaccinated?

    See the answer

    75%

    150 ÷ 200 × 100 = 75.

  11. About 95 in every 100 people need to be immune to stop a disease spreading. How many people is that in a town of 400?

    See the answer

    380 people

    95 ÷ 100 × 400 = 380.

Keep it

Copy this into your exercise book.

A vaccine holds a harmless form of a pathogen. White blood cells make antibodies and memory cells. The real pathogen then meets a fast, big response. Herd immunity protects people who cannot be vaccinated.

Draw: A row of people with the vaccinated ones shaded, and an arrow of infection that stops at the shaded people.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (1.0 MB). It opens on any phone.