What is vaccine efficacy (VE)?

VE measures the proportional reduction in disease risk among vaccinated individuals compared to unvaccinated individuals in a controlled trial. A VE of 95% means vaccinated participants had a 95% lower attack rate than the placebo group.

How is VE different from vaccine effectiveness?

"Efficacy" refers to results from randomized controlled trials under ideal conditions. "Effectiveness" refers to real-world observational data. Both use the same 1 - RR formula, but effectiveness estimates include additional confounders.

What is Relative Risk (RR)?

RR is the ratio of the attack rate in the vaccinated group to the attack rate in the unvaccinated group. An RR of 0.05 means vaccinated people have 5% of the risk of unvaccinated people, corresponding to a VE of 95%.

What is Number Needed to Vaccinate (NNV)?

NNV is 1 / ARR. It tells you how many people need to be vaccinated to prevent one additional case. A lower NNV indicates a greater absolute benefit per person vaccinated.

What if VE is negative?

A negative VE means the vaccinated group had a higher attack rate than the unvaccinated group. This can occur due to chance in small trials, unblinding issues, or rare adverse events. Check your inputs carefully.

Does this tool account for confidence intervals?

This calculator computes point estimates only. For 95% confidence intervals around VE, you would need additional statistical software or a binomial proportion CI method.

Vaccine Efficacy Calculator

Enter vaccinated and unvaccinated group sizes and case counts to instantly compute vaccine efficacy, relative risk, and absolute risk reduction.

Trial Data Input

Vaccinated Group

Total Participants

Cases (Outcomes)

Unvaccinated (Control) Group

Total Participants

Cases (Outcomes)

Load a published example:

Enter trial data on the left and click Calculate.

Vaccine Efficacy (VE)

—

Relative Risk (RR)

—

Vaccinated ÷ Control AR

Absolute Risk Reduction

—

Control AR − Vaccinated AR

Attack Rate — Vaccinated

—

Cases per 1,000 participants

Attack Rate — Control

—

Cases per 1,000 participants

Number Needed to Vaccinate (NNV)

—

Vaccinate this many people to prevent 1 additional case

Group Summary

Group	Participants	Cases	Attack Rate

Interpretation note: VE is a point estimate computed from trial totals. Statistical significance and confidence intervals require additional analysis. Small group sizes may yield unreliable estimates.

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Summary

This calculator uses the standard WHO/FDA formula for vaccine efficacy: VE = (1 - Relative Risk) × 100%. Relative Risk (RR) is the attack rate in the vaccinated group divided by the attack rate in the unvaccinated (placebo) group. A VE of 90% means vaccinated individuals have a 90% lower risk of the outcome compared to unvaccinated individuals. The tool also computes Absolute Risk Reduction (ARR) and Number Needed to Vaccinate (NNV).

How it works

Enter the number of participants in the vaccinated group and the number who developed the outcome.
Enter the number of participants in the unvaccinated (control/placebo) group and their case count.
The calculator computes attack rates for each group.
Relative Risk (RR) is computed as: attack rate (vaccinated) / attack rate (unvaccinated).
Vaccine Efficacy (VE) is then: (1 - RR) × 100%.
Absolute Risk Reduction (ARR) and Number Needed to Vaccinate (NNV) are displayed for additional context.

Use cases

Evaluate published clinical trial results for vaccines or prophylactic treatments.
Compare vaccine efficacy across different subgroups or variants.
Compute VE from press-release data before full papers are published.
Teach epidemiology concepts like relative risk and absolute risk reduction.
Quickly check if reported VE figures are consistent with the underlying trial data.
Estimate NNV to contextualize population-level benefit.

Frequently Asked Questions

Last updated: 2026-06-04 · Reviewed by Nham Vu