Calculate Herd Immunity Thresholds
Use hearing or immune-response evidence to calculate herd-immunity thresholds.
- Transmission basics: Outbreak work depends on agent, host, route, time, and place.
- Case definition: Students need a rule for who counts as a case before counting cases.
Prerequisites are inferred: pending teacher review.
Re-learn the skill with worked practice and clear examples.
Practice the calculation across diseases. The bigger the R0, the higher the threshold. Use the table to check your work.
Polio has R0 = 5 (see table). Using threshold = 1 - 1/R0, what percent of the population must be immune?
Reviewed| Disease | R0 | Threshold = 1 - 1/R0 |
|---|---|---|
| Flu | 2 | 0.50 (50%) |
| Polio | 5 | 0.80 (80%) |
| Measles | 10 | 0.90 (90%) |
- A.80%
- B.20%
- C.75%
- D.95%
Show the worked solution ▾
Answer: A. 80%
- Step 1: Divide: 1 / 5 = 0.20.
- Step 2: Subtract and convert: 1 - 0.20 = 0.80 = 80%.
Why it's right: 1 - 1/5 = 1 - 0.20 = 0.80, which is 80%.
- B: 20% is just 1/5 as a percent: you must subtract it from 100%.
- C: 75% is the answer for R0 = 4, not R0 = 5.
- D: 95% is the answer for R0 = 20, not R0 = 5.
Aligned to BRE: compute threshold for R0=5 · reading level ~grade 9
- A vaccination drive sets an 80% coverage goal for a disease with R0 = 5.
Fill these in as you work through the lesson.
- R0 (people one case infects in an unprotected group):
- Herd-immunity threshold (immune fraction that stops spread):
- Immune (protected, cannot pass it on):
- Contagious (how easily it spreads):
Herd-immunity threshold = 1 - (1 / ). Convert the decimal to a before comparing to a population.
- What is 1/R0 when R0 = 5?
- What is 1 - 1/5 as a percent?
- Why does a higher R0 need a higher threshold?
For R0 = 5, compute 1/R0, subtract from 1, and convert to a percent; then say whether a disease with R0 = 10 needs more or fewer people immune.