Vaccine Efficacy
Determine vaccine efficacy from clinical trial data using the standard WHO formula. Compare infection rates between vaccinated and unvaccinated groups with charts and detailed breakdown.
About This Calculator
The Vaccine Efficacy Calculator helps researchers, healthcare professionals, and students determine how well a vaccine prevents disease in clinical trial settings. By comparing infection rates between vaccinated and unvaccinated groups, you can calculate the standard vaccine efficacy measure used by the WHO, FDA, EMA, and other regulatory bodies worldwide.
The calculator uses the formula: VE = (ARU − ARV) / ARU × 100%, where ARU is the attack rate in unvaccinated participants and ARV is the attack rate in vaccinated participants. Results include vaccine efficacy percentage, infection rates, relative risk, and absolute risk reduction. The bar chart compares infection rates side by side, while the pie chart shows the distribution of total cases between groups.
How to Use
Enter the total number of people in the vaccinated and unvaccinated groups of your trial, then enter the number of confirmed infections in each group. The calculator automatically computes vaccine efficacy, which measures the proportional reduction in disease risk among vaccinated individuals compared to unvaccinated individuals.
Key Metrics Explained
- Vaccine Efficacy (VE): The percentage reduction in infection risk among vaccinated people compared to unvaccinated people.
- Infection Rates: The percentage of each group that contracted the disease during the trial period.
- Relative Risk (RR): The ratio of infection risk in vaccinated versus unvaccinated groups. VE = (1 − RR) × 100%.
- Absolute Risk Reduction (ARR): The absolute difference in infection rates between groups, representing the proportion of people directly saved by vaccination.
Regional Context
India (DCGI/CDSCO): The Drugs Controller General of India approves vaccines with minimum 50% efficacy. India's Covaxin and Covishield trials followed standard WHO efficacy protocols.
United States (FDA): The FDA requires at least 50% efficacy for vaccine approval with lower bound of 95% confidence interval above 30%. COVID-19 vaccines from Pfizer (95%) and Moderna (94%) met these criteria.
United Kingdom (MHRA): The MHRA/EMA requires similar standards. The Oxford-AstraZeneca vaccine showed 70% average efficacy in UK/Brazil trials.
Frequently Asked Questions
What is vaccine efficacy?
Vaccine efficacy measures how well a vaccine works in controlled clinical trial conditions. It is calculated as the percentage reduction in disease incidence among vaccinated people compared to unvaccinated people in the same trial. For example, if 10 out of 1,000 vaccinated people get infected while 90 out of 1,000 unvaccinated people get infected, the vaccine efficacy is (9% - 1%) / 9% = 88.9%.
How is vaccine efficacy calculated?
Vaccine efficacy is calculated using the formula: VE = (ARU - ARV) / ARU × 100%, where ARU is the attack rate in the unvaccinated group (cases divided by total unvaccinated) and ARV is the attack rate in the vaccinated group (cases divided by total vaccinated). This formula gives the percentage of disease cases prevented by vaccination.
What is the difference between vaccine efficacy and effectiveness?
Efficacy refers to how well a vaccine performs in controlled clinical trial conditions with carefully selected participants. Effectiveness refers to real-world performance across the general population, including people with pre-existing conditions, different ages, and varying exposure levels. Effectiveness is typically lower than efficacy and is measured after widespread deployment.
What does 95% vaccine efficacy mean?
A 95% vaccine efficacy means that vaccinated individuals have 95% fewer infections than unvaccinated individuals in the trial. For a practical example, if a population of 10,000 unvaccinated people would normally experience 500 infections (5% attack rate), only 25 vaccinated people would get infected (0.25% attack rate).
Can vaccine efficacy be negative?
Yes, vaccine efficacy can theoretically be negative if more vaccinated people get infected than unvaccinated people, suggesting the vaccine may increase infection risk. In practice, negative efficacy indicates the vaccine candidate is ineffective and requires further investigation. Our calculator caps efficacy at 0% to avoid misleading negative values.
How many participants are needed in a vaccine trial?
COVID-19 vaccine trials typically enrolled 30,000-50,000 participants split between vaccine and placebo groups. The required sample size depends on expected disease incidence, desired statistical power, and the efficacy threshold. Larger trials provide more precise efficacy estimates. Regulatory agencies like the FDA, EMA, and DCGI require minimum efficacy of 50% for approval.
What is relative risk in the context of vaccines?
Relative risk (RR) is the ratio of infection rates between vaccinated and unvaccinated groups. An RR of 0.11 means vaccinated people have 11% of the infection risk compared to unvaccinated people. Vaccine efficacy = (1 - RR) × 100%. So RR of 0.11 corresponds to 89% vaccine efficacy.
Is this calculator applicable to all vaccines?
Yes, the vaccine efficacy formula works universally for any vaccine where trial data with vaccinated and unvaccinated groups is available, including COVID-19, influenza, HPV, hepatitis B, and others. The calculator uses the standard WHO-recommended efficacy formula that applies across all disease areas and vaccine types.