efficacy

American  

[ef-i-kuh-see] / ˈɛf ɪ kə si /

Sometimes efficacity

noun

• efficacies
  plural

1. the capacity for producing a desired result or effect.

   Short, frequent periods of practice were shown to have greater efficacy than longer and less frequent ones.

2. a measure of the success of a vaccine or other pharmaceutical when used in the controlled environment of a clinical trial, as opposed to in the real world.

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efficacy British  

/ ˈɛfɪkəsɪ /

noun

1. the quality of being successful in producing an intended result; effectiveness

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Other Word Forms

Derived Forms

• nonefficacy
  noun

Inflected Forms

Nouns

• efficacies
  plural

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Etymology

Origin of efficacy

First recorded in 1520–30; from Latin efficācia, from efficāc- (stem of efficāx ) “effective, effectual” + -ia, noun suffix; see efficacious ( def. ), -y ³ ( def. )

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Explanation

The degree to which a method or medicine brings about a specific result is its efficacy. You might not like to eat it, but you can't question the efficacy of broccoli as a health benefit. Efficacy is a more formal way to say effectiveness, both of which stem from the Latin verb efficere "to work out, accomplish." The effectiveness, or efficacy, of something is how well it works or brings the results you hoped for. A scientist does research to determine the efficacy of a vaccine or medicine under development. If it is efficacious, it will cure or prevent a disease.

Example Sentences

See Examples For:

efficacy efficacies

Trying to explain the device, and how her claims of its efficacy were misread by investors and the public, Holmes once again can’t translate her thoughts into words that make sense to Fielder.

From Salon ● Oct. 17, 2026

"This lets us speak about safety and efficacy with real confidence," Dr. Fleckenstein said.

From Science Daily ● Oct. 14, 2026

Scientific evidence is lacking, and doctors and regulators have raised concerns about the widespread use, safety and efficacy of the substances.

From The Wall Street Journal ● Oct. 9, 2026

At the same time, he is ambivalent about the efficacy of outright bans on school devices.

From Los Angeles Times ● Sep. 14, 2026

Modern trials of the efficacy of therapies depend on the idea that an effective therapy must perform better than a placebo.

From "The Invention of Science" by David Wootton

For the next pandemic, if we have certain vaccines with starting efficacies, we can predict the minimum proportion we need to vaccinate to achieve vaccine-induced herd immunity.

From Scientific American ● Apr. 6, 2023

To get a little technical, the Pfizer data, in particular, had wide confidence intervals—basically, the efficacies are more like pretty good estimates, than numbers that the scientists are really certain of.

From Slate ● Jun. 22, 2022

That would make it still well above the 50% threshold needed for FDA authorization, but would pale in comparison to the 90-plus efficacies of other COVID-19 vaccines.

From Science Magazine ● Mar. 25, 2021

Further tests would be needed to judge how much the 2P mutation contributes to the overall efficacies of the frontrunner vaccines.

From National Geographic ● Dec. 31, 2020

That these two are different will easily be clear to anyone who passes in review their respective efficacies.

From The Consolation of Philosophy by H. R. (Henry Rosher) James

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Related Words

• adequacy
• competence
• effectiveness
• potency
• virtue

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