Vaccine Efficacy Calculator (VE from attack rates in vaccinated and unvaccinated groups)

Understanding the vaccine efficacy calculator — concept and principle

Vaccine efficacy (VE) is the percentage reduction in disease risk among vaccinated people compared with unvaccinated people. This calculator works it out from the number of cases and people in each group, as used in trials (efficacy) and outbreak investigations (effectiveness).

Compare attack rates: with the default inputs the result is VE 72.4% (1 − 0.276 risk ratio).

How it is calculated

Attack rate (risk) in each group = cases ÷ people in the group

Risk ratio = risk vaccinated ÷ risk unvaccinated

VE (%) = (risk unvaccinated − risk vaccinated) ÷ risk unvaccinated × 100 = (1 − risk ratio) × 100

The principle

If the vaccine had no effect, both groups would have the same risk and the ratio would be 1. A ratio below 1 means fewer cases among the vaccinated, and 1 minus the ratio is the share of risk removed. A VE of 72% means vaccinated people had 72% fewer cases than they would have had at the unvaccinated risk. The arithmetic is the same for trials and real-world studies; only the setting and possible biases differ.

Worked example

Default inputs: 18 cases among 152 vaccinated, 3 cases among 7 unvaccinated.

Attack rate vaccinated = 18 ÷ 152 = 11.8%.
Attack rate unvaccinated = 3 ÷ 7 = 42.9%.
Risk ratio = 0.1184 ÷ 0.4286 = 0.276.
VE = (1 − 0.276) × 100 = 72.4%.

How to read the result

Higher VE means stronger protection against the outcome measured. In this example the unvaccinated group is tiny, so the estimate is very uncertain. Groups must be comparable in exposure, age and testing, or the VE can be biased. A negative VE means more cases among the vaccinated, which usually signals bias or chance in small studies.

Common mistakes

Comparing groups with different exposure, such as healthcare workers versus others.
Ignoring that VE depends on the outcome (infection, illness, hospitalisation).
Reading VE as the chance a vaccinated person stays healthy.

Keep in mind: This is an epidemiology measure, not a diagnosis or a vaccination recommendation. Follow national immunisation guidance.

Frequently asked questions

What does 90% vaccine efficacy mean?

Vaccinated people had 90% fewer cases than unvaccinated people in the study, not that 10% of them got sick.

What is the difference between efficacy and effectiveness?

Efficacy comes from controlled trials; effectiveness from real-world use.

Why is a confidence interval important?

Small numbers of cases make the estimate uncertain; the interval shows the plausible range.

Sources: Orenstein WA, Bernier RH, Dondero TJ, et al., "Field evaluation of vaccine efficacy", Bull World Health Organ 1985;63(6):1055–1068 · Centers for Disease Control and Prevention, Principles of Epidemiology in Public Health Practice, 3rd ed., Lesson 3

Checked 9 Nov 2026. Centers for Disease Control and Prevention, Principles of Epidemiology in Public Health Practice, 3rd edition, Lesson 3 “Measures of Risk”, Section 6 “Measures of Public Health Impact”: VE = (risk among unvaccinated − risk among vaccinated) ÷ risk among unvaccinated, or 1 − risk ratio (varicella example: (42.9 − 11.8) ÷ 42.9 = 72%).

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