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

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

From Science Daily ● Oct. 14, 2026

“Another thing about emphasizing the efficacy of youthful endeavor in changing the world is the riff of the song was written by Roman,” Morello explained.

From Salon ● Sep. 22, 2026

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

From Los Angeles Times ● Sep. 14, 2026

Conscious feelings have more efficacy than nonconscious processes, meaning that once they emerged, they conferred so great an advantage that natural selection had to favor them.

From The Wall Street Journal ● Sep. 4, 2026

That’s the kind of fatalism and pessimism typical of a repressive feudal system, where peasants have no reason to believe in the efficacy of their own work.

From "Outliers" by Malcolm Gladwell

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

Still, the vaccines, shown in clinical trials to have efficacies of up to 95% against symptomatic disease, have finally given the world the prospect of an escape from COVID-19’s long siege.

From Science Magazine ● Feb. 16, 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

Along these lines, computational studies have shown that ligands with different efficacies modulate the free-energy landscape of the receptor by shifting the conformational equilibrium towards active or inactive conformations depending on their elicited physiological response.

From Nature ● Feb. 13, 2013

The believer in these processes thinks that certain acts possess particular efficacies beyond those evident to his observation and reason; and that peculiar malignities are to be expected as the consequence of certain other acts.

From Applied Eugenics by Paul Popenoe

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Related Words

• adequacy
• competence
• effectiveness
• potency
• virtue

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