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

Definition of efficacy

: the power to produce an effect

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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
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Recent Examples on the Web The amendments introduced many changes to the regulatory code, but the most striking one was this: For the first time, drug companies would be required to supply proof of efficacy. New York Times, "How Humanity Gave Itself an Extra Life," 27 Apr. 2021 University of Oxford scientists have reportedly developed the first malaria vaccine which, in a trial, surpassed a key goal of greater than 75 percent efficacy. TheWeek, "New malaria vaccine could 'have a major public health impact,' trial suggests," 23 Apr. 2021 White House Chief Medical Advisor Dr. Anthony Fauci is talking to AL.com on Friday in an effort to raise awareness of coronavirus vaccine efficacy in Alabama. Ivana Hrynkiw | Ihrynkiw@al.com, al, "Dr. Anthony Fauci takes COVID-19 vaccine questions from AL.com readers," 23 Apr. 2021 Chinese vaccine makers Sinopharm and Sinovac have presented data on their vaccines indicating levels of efficacy that would be compatible with those required by the WHO. USA Today, "Tracking COVID-19 vaccine distribution by state: How many people have been vaccinated in the US?," 22 Apr. 2021 This could imply a blunting of vaccine efficacy, but the reports from Israel suggest that at least one of the vaccines still offers some protection against it. Siladitya Ray, Forbes, "‘Double Mutant’ Covid Variant: Here’s What We Know As Israel Reports Vaccine Efficacy Against Variant First Detected In India," 20 Apr. 2021 The first phase of the Pfizer study is geared toward finding a safe dosage for preschool-age children, while the second and third phases will include a study of efficacy, Maldonado said. Hayley Smith, Los Angeles Times, "Stanford begins testing Pfizer COVID-19 vaccine on children as young as 2," 15 Apr. 2021 The company said the shot was 76% effective in its U.S. trial in a statement on Thursday, after an independent monitoring board expressed concern that its previous disclosure of 79% efficacy relied on outdated information. James Paton, Fortune, "AstraZeneca revises vaccine efficacy down to 76% after criticism of U.S. trial data," 25 Mar. 2021 An earlier study in South Africa yielded similar results, of only 10% efficacy. David Hogberg, Washington Examiner, "Lower AstraZeneca vaccine efficacy against South African strain could complicate pandemic response," 17 Mar. 2021

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.

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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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Time Traveler for efficacy

Time Traveler

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

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Statistics for efficacy

Last Updated

2 May 2021

Cite this Entry

“Efficacy.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/efficacy. Accessed 13 May. 2021.

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English Language Learners Definition of efficacy

formal : the power to produce a desired result or effect

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