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A vaccine works by imitating an infection. Its active ingredient is an antigen: weakened or killed germs, pieces of their surface, or a germ's poison (toxin) made harmless.

Some newer vaccines carry instructions (DNA or RNA) that tell the body's own cells to make the antigen.

The immune system reacts as it would to the real germ, but without the disease: white blood cells multiply and make antibodies, proteins that recognise that germ.

Experiment: Can vaccines stop a spread?

1. Use the same grid and the same starting square (the middle one) for every run: that keeps the test fair.
2. Run 1: no one is vaccinated. The starting square has 'caught it'. Each 'caught it' square passes it to every square that touches its sides, unless that square is vaccinated. Keep going until nothing new happens. Count the squares that caught it.
3. Run 2: roll the die for each square; a 5 or 6 means 'vaccinated' (about 1 in 3). Repeat the spread and count.
4. Run 3: now a 3, 4, 5 or 6 means 'vaccinated' (about 2 in 3). Repeat and count.
5. Do each run three times, find the mean and compare. What changed when more squares were vaccinated?

Run	How many vaccinated?	Squares that caught it (mean of 3)
Run 1: no one	Predict: which run will have the fewest squares catching it?
Run 2: about 1 in 3	Predict: which run will have the fewest squares catching it?
Run 3: about 2 in 3	Predict: which run will have the fewest squares catching it?

Questions

1. Explain in your own words how a vaccine prepares the body for a germ. Use: antigen, antibody, memory cell.

2. Why are some vaccines given in two or more doses?

3. What did your grid experiment show about vaccinating more people? Use your mean results.

4. Give one way the grid model is different from real life.

Sources for every fact on this sheet: echoesof.science/school/vaccines/sources/

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