Vaccine Efficacy Calculator

The Vaccine Efficacy Calculator estimates the percentage reduction in disease risk based on clinical trial infection data. This tool helps researchers and public health officials quickly determine how well a vaccine performs compared to a placebo. Whether you are evaluating a new candidate, analyzing real-world data, or preparing a study report, this tool provides precise statistical insights.

Number of people who got infected despite vaccination
Number of people who got infected without vaccination
Total number of people who received the vaccine
Total number of people who received placebo

How Vaccine Efficacy Is Calculated

Vaccine Efficacy (VE) = (1 - Relative Risk) × 100%

Where:

  • Relative Risk (RR) = (Cases in vaccinated / Total vaccinated) / (Cases in unvaccinated / Total unvaccinated)

First, the calculator determines the attack rate, or infection percentage, for both the vaccinated and unvaccinated groups. This step establishes the baseline risk for each population, which is crucial for accurate comparison.

Next, it calculates the Relative Risk by dividing the attack rate of the vaccinated group by that of the unvaccinated group. This ratio shows exactly how much lower the risk is for people who received the vaccine.

Finally, the calculator subtracts the Relative Risk from 1 and multiplies by 100 to convert the result into a percentage. This method, known as the standard epidemiological formula, ensures your results are consistent with scientific literature.

What Your Vaccine Efficacy Means

The result tells you the percentage drop in infection risk for the vaccinated group compared to the unvaccinated group. A higher percentage indicates stronger protection against the disease.

Interpreting Your Results

High Efficacy (90% or higher): This suggests the vaccine provides very strong protection and is highly effective at preventing infection in a trial setting.

Moderate Efficacy (50% to 89%): While lower than top-tier scores, this range is often considered acceptable for public health use, especially for severe disease prevention.

This calculator is for educational and research purposes only. It is not a substitute for professional medical advice or comprehensive clinical trial analysis. Always consult qualified healthcare professionals for decision-making.

Recalculate your results periodically as new data becomes available to monitor the vaccine’s performance against emerging variants or over time.