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.

---

efficacy British  

/ ˈɛfɪkəsɪ /

noun

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

---

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

He added efficacy also depended on the scale of the intervention, and that controlling populations this way risked becoming a "forever enterprise."

From Barron's ● Aug. 7, 2026

Staff excluded from the efficacy calculation patients who had RP1 injected in all of their tumors, but only from the numerator and not denominator.

From The Wall Street Journal ● Jul. 28, 2026

The Oxford team say a second small Ebola vaccine trial is due to begin soon in Uganda, followed by a bigger study, looking at efficacy, in DR Congo.

From BBC ● Jul. 24, 2026

Allowing compounding pharmacies to legally produce peptides isn’t a stamp of approval from the FDA about their safety or efficacy.

From MarketWatch ● Jul. 24, 2026

This discipline is of such efficacy that not only does it maintain those who were born princes, but it enables men of private station on many occasions to rise to that position.

From "The Prince" by Niccolò Machiavelli

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

But Pfizer and Moderna’s vaccines have efficacies of 95 percent and 94.5 percent, respectively, at preventing moderate to severe disease.

From Slate ● Mar. 5, 2021

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

He will inquire of art,—the empiric art,—and rude accident, what latent efficacies they have detected in her, what churlish secrets of hers they have wrung from her.

From The Philosophy of the Plays of Shakspere Unfolded by Delia Bacon

Related Words

• adequacy
• competence
• effectiveness
• potency
• virtue