# Vaccine Efficacy Calculator

URL: https://speedycalculator.com/calculators/health/vaccine-efficacy/
Last updated: 2026-10-30

> The vaccine efficacy calculator works out how much a vaccine reduces the risk of disease, from attack rates in vaccinated and unvaccinated groups, along with the relative risk and the number needed to vaccinate.

## Inputs

- Cases in the unvaccinated group
- People in the unvaccinated group
- Cases in the vaccinated group
- People in the vaccinated group

## Results

- Attack rate – unvaccinated
- Attack rate – vaccinated
- Vaccine efficacy
- Relative risk
- Number needed to vaccinate

## Formula

```
Attack rate = cases ÷ people in the group
Vaccine efficacy = (ARU − ARV) ÷ ARU × 100 = (1 − RR) × 100
Number needed to vaccinate = 1 ÷ (ARU − ARV)
```

## Explanation

Efficacy is measured in controlled trials; effectiveness is measured in real-world use. A vaccine with 90% efficacy prevents 90% of cases that would otherwise have happened in vaccinated people.

### How to use the Vaccine Efficacy Calculator

- **Cases in the unvaccinated group**
- **People in the unvaccinated group**
- **Cases in the vaccinated group**
- **People in the vaccinated group**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Enter the number of cases and people in each group.

### Tips

- Use counts and person-time from the same population and period.
- Report confidence intervals with rates and ratios.
- Check whether figures are crude or age-adjusted before comparing.
- Larger samples give more stable estimates.

### Common mistakes to avoid

- Confusing incidence (new cases) with prevalence (all cases).
- Comparing rates per 1,000 with rates per 100,000.
- Reading a relative risk without the baseline risk.

### Inputs explained

FieldWhat to enterCases in the unvaccinated groupA numberPeople in the unvaccinated groupA numberCases in the vaccinated groupA numberPeople in the vaccinated groupA number

### What each result means

- **Attack rate – unvaccinated** = [Cases in the unvaccinated group]÷[People in the unvaccinated group]×100
- **Attack rate – vaccinated** = [Cases in the vaccinated group]÷[People in the vaccinated group]×100
- **Vaccine efficacy** = (1 - [Attack rate – vaccinated]÷[Attack rate – unvaccinated])×100
- **Relative risk** = [Attack rate – vaccinated]÷[Attack rate – unvaccinated]
- **Number needed to vaccinate** = 100÷([Attack rate – unvaccinated] - [Attack rate – vaccinated])

### Example values

How the main result changes when **Cases in the unvaccinated group** changes and everything else stays at the example values above:

Cases in the unvaccinated groupVaccine efficacy (%)50807586.7100901259215093.320095
*Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.*

## Worked example

Inputs — Cases in the unvaccinated group: 100; People in the unvaccinated group: 10,000; Cases in the vaccinated group: 10; People in the vaccinated group: 10,000.

Results — Attack rate – unvaccinated = 1 %; Attack rate – vaccinated = 0.1 %; **Vaccine efficacy = 90 %**; Relative risk = 0.1; Number needed to vaccinate = 111.

## Key terms

- **Attack rate**: Proportion of a group that develops the disease.

## FAQ

**What is the difference between efficacy and effectiveness?**
Efficacy is measured in trials; effectiveness in everyday use.

**Does 95% efficacy mean 5% of vaccinated people get sick?**
No – it means a 95% lower risk compared with unvaccinated people.

**What is the number needed to vaccinate?**
How many people must be vaccinated to prevent one case.


## Sources

- [Orenstein WA et al. Field evaluation of vaccine efficacy. Bull World Health Organ, 1985](https://pubmed.ncbi.nlm.nih.gov/3879673/)