π‘οΈ Vaccine Efficacy Calculator
The percentage behind every trial headline.
Vaccine efficacy
For informational purposes only, not medical advice.
Vaccine efficacy is the percentage reduction in disease: (attack rate in unvaccinated minus attack rate in vaccinated) divided by the attack rate in unvaccinated. With 100 cases among 1,000 unvaccinated (10%) and 20 among 1,000 vaccinated (2%), efficacy is (10 - 2) / 10 = 80%. It does not mean 20 percent of vaccinated people got sick; it means their risk fell by 80 percent relative to the unvaccinated.
That relative framing is the most misunderstood number in public health. Ninety-five percent efficacy with a 1 percent background rate means the vaccinated risk is 0.05 percent, not 5 percent. The percentage describes the reduction, not the remainder.
How to Use This Calculator
Enter the case counts and group sizes from a trial's two arms. The tool computes both attack rates and the efficacy between them.
Enter cases and total participants in the unvaccinated (placebo) arm.
Enter cases and total participants in the vaccinated arm.
Read the attack rates and the efficacy percentage with its plain-English meaning.
The Efficacy Formula
Efficacy is measured against a specific endpoint: symptomatic disease, severe disease, or infection, and the number changes with the endpoint. A vaccine can be 95 percent effective against severe disease but 60 percent against any infection, and both numbers are true simultaneously. Headlines that omit the endpoint mislead even when the arithmetic is right.
Effectiveness is efficacy's real-world sibling, measured in observational studies rather than trials, and usually a bit lower because real life includes imperfect storage, delayed doses, and sicker populations. Efficacy is the trial ideal; effectiveness is the field result.
Worked Example
Placebo arm: 100 cases in 1,000 people, a 10 percent attack rate. Vaccine arm: 20 cases in 1,000, a 2 percent attack rate. Efficacy is (10 - 2) / 10 x 100 = 80%. Twenty vaccinated people still got sick, which anti-vaccine rhetoric sometimes parades as failure, but their risk was one-fifth of the alternative.
Absolute terms sharpen it: the vaccine prevented 80 cases per 1,000 vaccinated, so about 13 people need vaccinating to prevent one case. Relative efficacy impresses; absolute benefit persuades.
Reading Your Result
| Efficacy | Context |
|---|---|
| 90%+ | Excellent. Most childhood vaccines live here. |
| 70-90% | Very good. Typical of strong adult vaccines. |
| 50-70% | Moderate. Worthwhile at population scale; flu vaccines often land here. |
| Under 50% | Modest against that endpoint; may still prevent severe outcomes well. |
My take: judge efficacy against the disease's stakes, not against 100. A 60 percent effective vaccine against a common severe illness saves more lives than a 95 percent one against a rare mild one. The percentage is half the story; the disease is the other half.
Limits
Trial efficacy reflects the trial's population, variant, and time window; waning, new variants, and different age groups all shift it. Confidence intervals matter: small trials produce wide ones. And efficacy against infection fades faster than efficacy against severe disease for most vaccines, so specify the endpoint or say nothing.
Related Tools
See how long protection lasts with our vaccine immunity guide and what the US schedule recommends in the US vaccine schedule checker.
For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
How is vaccine efficacy calculated?
(Attack rate in unvaccinated - attack rate in vaccinated) / attack rate in unvaccinated x 100. With 10% vs 2% attack rates, efficacy is 80%. It measures relative risk reduction in a trial.
What is the difference between efficacy and effectiveness?
Efficacy comes from randomized trials under ideal conditions; effectiveness from real-world observational studies. Effectiveness is usually somewhat lower due to imperfect storage, timing, and broader populations.
Does 95% efficacy mean 5% get sick?
No. It means vaccinated people's risk fell 95% relative to unvaccinated. If the background rate was 1%, the vaccinated rate is 0.05%. The percentage is the reduction, not the remainder.
Why do efficacy numbers change over time?
Waning immunity, new variants, different populations, and different endpoints (infection vs severe disease) all shift the number. A vaccine's efficacy is a snapshot, not a permanent property.
What is a good vaccine efficacy?
It depends on the disease: 50%+ against a common severe illness is a major public health win (typical flu range), while childhood vaccines often exceed 90%. Judge against the stakes, not against perfection.
What is NNV?
Number needed to vaccinate: how many people must be vaccinated to prevent one case, equal to 1 / absolute risk reduction. It translates efficacy into concrete population impact.
Can efficacy be negative?
In observational data, apparent negative effectiveness can appear from biases like frailty or behavior differences between groups. In randomized trials, true negative efficacy would mean harm and would halt the trial.
Why do trials use placebo groups?
To isolate the vaccine's effect from background infection rates and behaviors. Attack rates in both arms face the same exposures, so their ratio measures the vaccine alone.
Does efficacy measure side effects?
No. Efficacy is about disease prevention; safety is assessed separately through adverse event monitoring in trials and post-marketing surveillance. Both matter; they are different numbers.
Where can I find official efficacy data?
The CDC, WHO, and published trial papers report efficacy by endpoint with confidence intervals. This calculator teaches the arithmetic; official sources provide the verified numbers.