Efficacy is measured in controlled trials; effectiveness is measured in real-world use. A vaccine with 90% efficacy prevents 90% of cases that would otherwise have happened in vaccinated people.
How to use the Vaccine Efficacy Calculator
- Cases in the unvaccinated group
- People in the unvaccinated group
- Cases in the vaccinated group
- People in the vaccinated group
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the number of cases and people in each group.
Tips
- Use counts and person-time from the same population and period.
- Report confidence intervals with rates and ratios.
- Check whether figures are crude or age-adjusted before comparing.
- Larger samples give more stable estimates.
Common mistakes to avoid
- Confusing incidence (new cases) with prevalence (all cases).
- Comparing rates per 1,000 with rates per 100,000.
- Reading a relative risk without the baseline risk.
Inputs explained
| Field | What to enter |
|---|---|
| Cases in the unvaccinated group | A number |
| People in the unvaccinated group | A number |
| Cases in the vaccinated group | A number |
| People in the vaccinated group | A number |
What each result means
- Attack rate – unvaccinated = [Cases in the unvaccinated group]÷[People in the unvaccinated group]×100
- Attack rate – vaccinated = [Cases in the vaccinated group]÷[People in the vaccinated group]×100
- Vaccine efficacy = (1 – [Attack rate – vaccinated]÷[Attack rate – unvaccinated])×100
- Relative risk = [Attack rate – vaccinated]÷[Attack rate – unvaccinated]
- Number needed to vaccinate = 100÷([Attack rate – unvaccinated] – [Attack rate – vaccinated])
Example values
How the main result changes when Cases in the unvaccinated group changes and everything else stays at the example values above:
| Cases in the unvaccinated group | Vaccine efficacy (%) |
|---|---|
| 50 | 80 |
| 75 | 86.7 |
| 100 | 90 |
| 125 | 92 |
| 150 | 93.3 |
| 200 | 95 |
Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.
Formula
Attack rate = cases ÷ people in the group
Vaccine efficacy = (ARU − ARV) ÷ ARU × 100 = (1 − RR) × 100
Number needed to vaccinate = 1 ÷ (ARU − ARV)
Worked example
Inputs — Cases in the unvaccinated group: 100; People in the unvaccinated group: 10,000; Cases in the vaccinated group: 10; People in the vaccinated group: 10,000.
Results — Attack rate – unvaccinated = 1 %; Attack rate – vaccinated = 0.1 %; Vaccine efficacy = 90 %; Relative risk = 0.1; Number needed to vaccinate = 111.
Key terms
- Attack rate
- Proportion of a group that develops the disease.
Frequently asked questions
What is the difference between efficacy and effectiveness?
Efficacy is measured in trials; effectiveness in everyday use.
Does 95% efficacy mean 5% of vaccinated people get sick?
No – it means a 95% lower risk compared with unvaccinated people.
What is the number needed to vaccinate?
How many people must be vaccinated to prevent one case.