Epidemiologic metrics
Vaccine Efficacy & Effectiveness Estimator
Enter case counts and sample sizes for vaccinated versus placebo (or unvaccinated) cohorts to compute vaccine efficacy (VE) with confidence intervals. Supports continuity corrections when zero cases are observed in a group.
Default 0.95 = 95% confidence interval.
Attack rate vaccinated
0.1%
Attack rate placebo
1.1%
Vaccine efficacy (VE)
95.1%
CI: 90.0% – 97.6%
Relative risk (RR)
0.049
VE = 1 − RR.
Absolute risk reduction (ARR)
1.0%
Number needed to vaccinate ≈ 98
How to Use This Calculator
Collect case counts
Enter the number of symptomatic or confirmed infections among vaccinated and placebo (or unvaccinated) participants.
Provide denominators
Use total person counts per group; for observational studies, substitute person-years or exposures if appropriate.
Interpret VE and confidence interval
VE represents relative risk reduction. The confidence interval communicates statistical precision around the point estimate.
Formula
Attack rate (AR) = Cases ÷ Total participants (per group)
Relative risk (RR) = ARvaccinated ÷ ARplacebo
Vaccine efficacy = 1 − RR
SE(log RR) ≈ √[(1/Cv) − (1/Nv) + (1/Cp) − (1/Np)]
CIRR: exp(log RR ± z × SE), CIVE: 1 − CIRR
Absolute risk reduction = ARplacebo − ARvaccinated; NNV = 1 ÷ ARR
Full Description
Vaccine efficacy describes the proportional reduction in disease among vaccinated participants compared with a placebo or unvaccinated control. Randomized controlled trials typically report VE alongside 95% confidence intervals derived from binomial or time-to-event models. This calculator uses a simple risk ratio approach suitable for quick analyses and teaching examples.
For observational effectiveness studies, adjust denominators for person-time and control for confounding factors through regression. When either arm records zero cases, a continuity correction prevents division by zero but slightly widens intervals.
Frequently Asked Questions
Can I use this for real-world effectiveness studies?
Yes, if you substitute person-time denominators and ensure comparable groups. For sophisticated analyses, use regression or survival models.
Why apply a continuity correction?
When zero cases occur in one arm, RR is undefined. Adding 0.5 cases and 1 participant avoids mathematical issues while minimally affecting estimates.
How should I interpret negative VE values?
Negative VE indicates higher risk among vaccinated participants. Investigate data quality, variant escape, or confounding factors.
Does VE equal vaccine effectiveness?
Efficacy refers to randomized trial results under ideal conditions. Effectiveness measures performance in real-world deployments; calculations are similar but context differs.