What vaccine efficacy means
Vaccine efficacy measures the percentage reduction in disease risk among vaccinated people compared to unvaccinated people under controlled study conditions, such as a clinical trial.
The formula used
This calculator uses the standard epidemiological formula: Efficacy = (1 − (Attack Rate in Vaccinated ÷ Attack Rate in Unvaccinated)) × 100%, where attack rate is the proportion of each group that developed the disease.
Real-world vaccine effectiveness can differ from trial efficacy due to population differences, variant strains, and time since vaccination. This calculator is for general educational purposes and does not represent guidance on any specific vaccine or medical decision.
Last reviewed August 2026