Last updated October 10, 2026 · Report an error
Computes vaccine efficacy using the standard published epidemiological statistics formula used in clinical trial reporting: Vaccine Efficacy (%) = (1 - Relative Risk) x 100, where Relative Risk = attack rate vaccinated / attack rate unvaccinated. Pure arithmetic on the two given attack rates. For general reference and educational purposes only — this computes the standard trial-reporting statistic and is not a safety or effectiveness claim about any specific vaccine.
How it works
Enter the attack rate (percentage who got the disease) in the unvaccinated group and the vaccinated group. The tool computes the relative risk and vaccine efficacy.
- Enter attack rate — unvaccinated group (%).
- Enter attack rate — vaccinated group (%).
- Click Calculate to see your results.
Examples
Unvaccinated 10%, Vaccinated 1%
Relative Risk = 1/10 = 0.1. Vaccine Efficacy = (1 - 0.1) x 100 = 90%.
Who should use it
- Epidemiology and public health statistics education.
- Understanding how vaccine trial results are reported.
Industry applications
- Public health education
- Epidemiology coursework
Advantages
- Standard formula used in every vaccine clinical trial report.
- Simple, transparent arithmetic.
Limitations
- Does not account for confidence intervals or study design factors.
- Not a real-world safety or effectiveness endorsement.
Common mistakes to avoid
- Confusing vaccine efficacy (relative risk reduction) with absolute risk reduction, which is a different (usually smaller) number.
Best practices
- Report vaccine efficacy alongside the underlying attack rates and study conditions for full context, per standard epidemiological reporting practice.
Tips
- Vaccine efficacy is a relative measure — a 90% efficacy against a rare disease still prevents fewer absolute cases than the same efficacy against a common one, per standard epidemiological teaching.