Herd immunity
Choose a disease, from flu to measles, and how many people in a town are vaccinated. See how many people each case infects now, how many must be vaccinated for herd immunity, and whether the disease dies out or keeps spreading.
Core concept
Each case of a disease infects R₀ people when no one is immune; when more than 1 − 1/R₀ of people are vaccinated, each case infects fewer than one other and the disease dies out, protecting even those who cannot be vaccinated
Topic: Our body and health
When a sick person meets people who are not immune, they pass the germ on. The basic reproduction number, R₀ (R-nought), is how many people one case infects in a town where nobody is immune: about 15 for measles, 6 for polio, 5 for mumps, 3 for the first COVID-19 and 2 for flu. A vaccine makes people immune. If a share of the town is vaccinated, each case can only infect the rest, so the effective R is R₀ × (1 − the share vaccinated). When R falls below 1, each case passes the germ to fewer than one other, and the outbreak dies out. That needs more than 1 − 1/R₀ of the people vaccinated: herd immunity, which also shields babies and sick people who cannot have the vaccine.
What you will learn
- Explain what R₀ and the effective R mean.
- Work out the vaccination needed for herd immunity, 1 − 1/R₀.
- Explain why very infectious diseases like measles need almost everyone vaccinated.
Before you start
- Know that germs cause communicable diseases and that vaccines make us immune.
Materials
- Your own vaccination card, or a child's (ask at home)
Equipment
- None
Procedure
- Record measles with 90% of people vaccinated.
- Raise the vaccination to 95% and record.
- Choose polio at 90%, then flu at 50%, and record each.
How this simulation is simplified
- R₀: measles 15, polio 6, mumps 5, COVID-19 (first variant) 3, flu 2. These are typical textbook values; real ones vary.
- The vaccine is taken to protect fully, and people mix evenly in the town.