What this calculator does
Vaccine efficacy works out percentage reduction in disease risk among a vaccinated group versus an unvaccinated group. Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
FormulaEfficacy % = (attack rate unvaccinated − attack rate vaccinated) / attack rate unvaccinated × 100
The inputs explained
| Field | What to enter |
|---|---|
| Unvaccinated attack rate (%) | A number, measured in %. Starts at 5. |
| Vaccinated attack rate (%) | A number, measured in %. Starts at 1. |
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the answer changes with unvaccinated attack rate
Every other input is held at the calculator’s starting values while unvaccinated attack rate varies. Select any row to load that scenario into the calculator.
How the answer changes with unvaccinated attack rate
| Unvaccinated attack rate (%) | Vaccine efficacy | Relative risk (vaccinated vs. not) | Note |
|---|---|---|---|
| 2.5 | 60.0% | 0.40 | Calculated from attack rates in a trial or outbreak; observational real-world "effectiveness" studies use the same formula. |
| 3.75 | 73.3% | 0.27 | Calculated from attack rates in a trial or outbreak; observational real-world "effectiveness" studies use the same formula. |
| 5 | 80.0% | 0.20 | Calculated from attack rates in a trial or outbreak; observational real-world "effectiveness" studies use the same formula. |
| 7.5 | 86.7% | 0.13 | Calculated from attack rates in a trial or outbreak; observational real-world "effectiveness" studies use the same formula. |
| 10 | 90.0% | 0.10 | Calculated from attack rates in a trial or outbreak; observational real-world "effectiveness" studies use the same formula. |
| 15 | 93.3% | 0.07 | Calculated from attack rates in a trial or outbreak; observational real-world "effectiveness" studies use the same formula. |