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 But neither vaccine’s efficacy plummeted to zero—not even close. Katherine J. Wu, The Atlantic, "The Virus Is Evolving. But So Are Your Antibodies," 12 Feb. 2021 Even with the two-dose vaccine bumped up to 95 percent efficacy, their model suggested that efficiency remained key. Gregory Barber, Wired, "More Covid Vaccine Choices Mean New Equity Challenges," 5 Feb. 2021 On Monday, Mexican officials spoke with their Russian counterparts to secure 24 million doses of the Sputnik V vaccine, but health experts caution that the Russian vaccine’s efficacy and safety cannot be guaranteed. Oscar Lopez, BostonGlobe.com, "Mexicans worry that even getting COVID won’t make their leader take it seriously," 26 Jan. 2021 Studies with thousands of participants have shown the Pfizer and Moderna vaccines have 95 percent efficacy. al, "Only 1 in 6 low-income Blacks or Latinos in Alabama want a vaccine, study finds," 14 Jan. 2021 But the announcement on the vaccine’s efficacy, which had been expected this past Wednesday, was delayed until January. Ryan Dube, WSJ, "First Covid-19 Vaccines Are Given in Latin America, But Region Faces Challenges," 25 Dec. 2020 Eighty-six percent efficacy is just behind the rates reported by frontrunners Pfizer and Moderna, who say their jabs stop 94% and 95% of COVID-19 infections, respectively. Grady Mcgregor, Fortune, "UAE says China’s state-backed COVID-19 vaccine is 86% effective," 9 Dec. 2020 The vaccine’s efficacy and safety will have to be evaluated for each age group, and testing hasn’t started for infants, toddlers or kids in the U.S. Wesley Kufel, The Conversation, "When can children get the COVID-19 vaccine? 5 questions parents are asking," 8 Dec. 2020 Data previously released from large-scale clinical trials showed that the vaccine had nearly 95 percent efficacy and people who were immunized displayed no serious side effects. Washington Post, "Britain grants Pfizer coronavirus vaccine emergency authorization," 2 Dec. 2020

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

19 Feb 2021

Cite this Entry

“Efficacy.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/efficacy. Accessed 26 Feb. 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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