Vaccine Efficacy Calculator

Calculate vaccine efficacy from vaccinated and unvaccinated group sizes and infection counts.

1.0M uses Updated · 2026-05-25 Runs locally · zero upload
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How to Use the Vaccine Efficacy Calculator

Enter total people and infected people for both vaccinated and unvaccinated groups. Infections must not exceed the group total, and both groups should come from comparable conditions.

The calculator computes attack rates, relative risk, and vaccine efficacy. Positive efficacy means lower risk in the vaccinated group; negative efficacy means the vaccinated attack rate is higher.

Formula and Method - Vaccine Efficacy Calculator

Vaccine efficacy (%) = (1 - vaccinated attack rate ÷ unvaccinated attack rate) × 100

Vaccine efficacy expresses relative risk reduction. Lower vaccinated attack rate and higher unvaccinated attack rate produce higher efficacy. Bias can occur if exposure or baseline risk differs between groups.

Use Cases for the Vaccine Efficacy Calculator

  • Recalculating efficacy from trial or observational data.
  • Teaching the link between attack rate, relative risk, and efficacy.
  • Comparing rough efficacy patterns across time periods or subgroups.

Frequently asked questions about Vaccine Efficacy Calculator

What is an attack rate?

It is infected people divided by total people in a group.

Why can unvaccinated cases not be zero?

A zero unvaccinated attack rate makes the attack-rate ratio undefined.

Is efficacy an individual protection probability?

No. It is a group-level relative risk reduction measure.