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

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. )

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

Opponents have repeatedly questioned the law's viability and raised concerns over privacy, the efficacy of age-verification tools, the risks of young people bypassing them, and how quickly the ban has been designed and brought in.

From BBC ● Aug. 14, 2026

While the Dalai Lama expressed sorrow and questioned the efficacy of such protests, some scholars criticized him for not trying to stop the practice.

From Barron's ● Aug. 7, 2026

A spokesman added: "All pet parasite treatments undergo rigorous scientific assessment by independent regulatory authorities before approval for use, and their safety and efficacy continue to be monitored."

From BBC ● Aug. 5, 2026

But some analysts had pointed out amlitelimab’s efficacy didn’t beat Sanofi’s existing treatment Dupixent, the group’s bestselling product.

From The Wall Street Journal ● Jul. 24, 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

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

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 benignant efficacies of Concealment," cries our Professor, "who shall speak or sing?

From Sartor Resartus: the life and opinions of Herr Teufelsdröckh by Thomas Carlyle

Related Words

• adequacy
• competence
• effectiveness
• potency
• virtue