Calculator Inputs
Enter event counts and totals for vaccinated and unvaccinated groups. Results appear above this form after submission.
Plotly Graph
Example Data Table
| Study | Vaccinated Cases | Vaccinated Total | Unvaccinated Cases | Unvaccinated Total | Estimated VE |
|---|---|---|---|---|---|
| Example Trial A | 5 | 1000 | 25 | 1000 | 80.00% |
| Example Trial B | 18 | 2500 | 54 | 2500 | 66.67% |
| Example Trial C | 9 | 800 | 30 | 800 | 70.00% |
Formula Used
ARv = Vaccinated Cases / Vaccinated Total
ARu = Unvaccinated Cases / Unvaccinated Total
RR = ARv / ARu
VE = (1 − RR) × 100
ARR = ARu − ARv
NNV = 1 / ARR
log(RR) ± z × SE(log(RR)), then exponentiate both limits.
SE(log(RR)) = √[(1/a) − (1/n1) + (1/c) − (1/n0)]
How to Use This Calculator
- Enter a study label for easier export tracking.
- Type the number of cases observed in the vaccinated group.
- Enter the total vaccinated participants included in analysis.
- Type the number of cases observed in the unvaccinated group.
- Enter the total unvaccinated participants.
- Choose a confidence level and preferred decimal precision.
- Keep continuity correction enabled when zero cells may affect interval estimates.
- Click Calculate to view the results, table, and graph.
- Use the CSV or PDF buttons to export the output.
Frequently Asked Questions
1. What does vaccine efficacy measure?
Vaccine efficacy measures the relative reduction in disease risk among vaccinated participants compared with unvaccinated participants under the study conditions.
2. Is vaccine efficacy the same as effectiveness?
No. Efficacy usually comes from controlled trials, while effectiveness often reflects real-world performance after rollout in broader populations.
3. Why can efficacy be negative?
A negative value means the vaccinated group showed a higher observed attack rate than the unvaccinated group in that dataset.
4. Why does the calculator show relative risk too?
Relative risk is the core ratio behind efficacy. It helps users interpret the size of the underlying risk comparison directly.
5. What is absolute risk reduction?
Absolute risk reduction shows the direct difference between unvaccinated and vaccinated attack rates. It is useful for practical impact assessment.
6. What is number needed to vaccinate?
It estimates how many people need vaccination to prevent one additional case, based on the observed absolute risk reduction.
7. Why use a continuity correction?
Zero cells can break logarithmic interval formulas. A small correction stabilizes the estimate when counts are sparse.
8. Can this calculator replace full epidemiological analysis?
No. It is useful for rapid estimation, but full interpretation may also require stratification, adjustment, design review, and context.