Vaccine Efficacy Calculator

Work out vaccine efficacy instantly with clear inputs, formula shown and shareable results.

About Vaccine Efficacy Calculator

Compares attack rates between vaccinated and unvaccinated groups to give efficacy, and converts the absolute difference into a number needed to vaccinate.

Formula

Vaccine efficacy

VE (%) = (attack rate unvaccinated - attack rate vaccinated) / attack rate unvaccinated x 100 = (1 - relative risk) x 100

Worked example

With the default values below, Vaccine Efficacy Calculator returns Vaccine efficacy of 80.00%. Change any input above and every figure here is recalculated in your browser.

Inputs used

Attack rate in the unvaccinated groupaccepts 0.01–100: 8
Attack rate in the vaccinated groupaccepts 0–100: 1.6

Results

Vaccine efficacy: 80.00%
Relative risk in the vaccinated: 0.2000
Number needed to vaccinate: 15.6
Absolute risk reduction: 6.400%

Step by step

1Values used
2Vaccine efficacy
3Vaccine efficacy= 80.00
4Relative risk in the vaccinated= 0.2000
5Number needed to vaccinate= 15.6
6Absolute risk reduction= 6.400

Assumptions and limits

The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

What is the difference between efficacy and effectiveness?

Efficacy comes from trial conditions; effectiveness is measured in routine use where storage, timing and uptake vary.

Why does the number needed to vaccinate matter?

High efficacy against a rare outcome can still mean many people must be vaccinated to prevent one case.