ef·​fi·​ca·​cy | \ ˈe-fi-kə-sē How to pronounce efficacy (audio) \
plural efficacies

Definition of efficacy

: the power to produce an effect

Examples of efficacy in a Sentence

In the planner's view, Rumsfeld had two goals: to demonstrate the efficacy of precision bombing and to "do the war on the cheap." — Seymour M. Hersh, New Yorker, 7 Apr. 2003 Vaccines exist, but their efficacy against aerosolized plague is unknown. — Sharon Begley et al., Newsweek, 8 Oct. 2001 efficacy does not have to be demonstrated before homeopathic products are marketed. — Alison Abbott et al., Nature, 26 Sept. 1996 questioned the efficacy of the alarms in actually preventing auto theft
Recent Examples on the Web The sheer efficacy of vaccination is scientifically incontrovertible, and after clean water, immunization is the most life-saving intervention in human history. David Robert Grimes, Scientific American, 28 Mar. 2022 Moderna said that the efficacy was on par with what would be expected of a two-dose vaccine in adults against the Omicron variant, which predominated during the trial. Helen Branswell And Matthew Herper, STAT, 25 Mar. 2022 For children ages 6 months through 1 year old, the efficacy was 43.7%. John Bonifield, CNN, 23 Mar. 2022 The researchers estimated that the efficacy against any infection (asymptomatic and symptomatic) was 30 percent for the fourth Pfizer dose, and 11 percent for Moderna. Beth Mole, Ars Technica, 18 Mar. 2022 The mentally ill will live normal lives in the community, thanks to supports — social, employment, recreational, housing — and access to treatment, whose efficacy is, by the way, rapidly improving. Stephen Eide, National Review, 6 Mar. 2022 The results were gathered between Aug. 19 and Dec. 15, when delta was still dominant in the U.S., and efficacy against omicron is likely to be lower, the researchers said. Bloomberg.com, 26 Jan. 2022 In a preliminary analysis of lab tests collected during the Omicron wave, Moderna says its vaccine efficacy against infection was 51% among children younger than 2 and 37% among children from 2 to 5 years old. Alexander Tin, CBS News, 28 Apr. 2022 While these pills don’t visibly go bad, their efficacy can really drop. Caroline Delbert, Popular Mechanics, 27 Apr. 2022 See More

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'efficacy.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

First Known Use of efficacy

13th century, in the meaning defined above

History and Etymology for efficacy

borrowed from Latin efficācia, from efficāc-, efficāx "capable of fulfilling a function, efficacious" + -ia -y entry 2

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The first known use of efficacy was in the 13th century

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Last Updated

14 May 2022

Cite this Entry

“Efficacy.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/efficacy. Accessed 24 May. 2022.

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More from Merriam-Webster on efficacy

Nglish: Translation of efficacy for Spanish Speakers

Britannica English: Translation of efficacy for Arabic Speakers

Britannica.com: Encyclopedia article about efficacy


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