How this calculator works
Compare risks, not just case counts, because group sizes may differ. The scenario is a simple direct-effect calculation with a common baseline risk; it omits transmission effects and waning. This tool contains no historical COVID-19 product rates. The estimate refers only to the study outcome, population and follow-up period entered. It does not include a confidence interval or adjust confounding. The result is an estimate for information only and is not medical advice.
Formula
Risk = cases/group size. VE = 1 - vaccinated risk/control risk. Expected cases without vaccination = population x baseline risk; with vaccination = baseline cases x (1 - VE).
Worked example
If 100 of 1000 controls and 20 of 1000 vaccinated participants have the outcome, risks are 10% and 2%, so efficacy is 80%. In a 10,000-person scenario with 10% baseline risk, that implies 1000 versus 200 cases.
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Frequently asked questions
Does 80% efficacy mean 20% of recipients become ill?
No. It is a relative reduction. The absolute risk depends on the baseline risk and follow-up period.
Why are there no vaccine brand presets?
Efficacy depends on outcome, variant, time and population. Enter a study estimate appropriate to the question.
Can efficacy be negative?
Yes. The arithmetic is negative if vaccinated risk exceeds control risk. Sampling error or bias may explain an observed difference.