efficacy
noun
ef·fi·ca·cy
ˈe-fi-kə-sē 
plural efficacies
: the power to produce a result or effect
increased teacher efficacy in classroom management
The efficacy of this treatment has not yet been proved.
In the upcoming … clinical trial, researchers will further investigate the efficacy and safety of the vaccine.—
Julia Landwehr
Julia LandwehrExamples 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
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
Earlier this week, pharmaceutical company Pfizer and its partner Valneva announced that an experimental Lyme disease vaccine showed more than 70% efficacy in late-stage clinical trials.
—Mary Kekatos, ABC News, 24 Mar. 2026
Finding passions, hobbies, community and values outside of family can help build self-worth and self-efficacy, Cole said.
—Rachel Hale, USA Today, 24 Mar. 2026
Still, the company said the shot reduced the rate of infection by more than 70% in people who received the vaccine versus placebo, efficacy the company thinks is strong enough to take to regulators.
—Angelica Peebles, CNBC, 23 Mar. 2026
Pfizer and Valneva’s Lyme disease vaccine cut cases by about 73% in a Phase 3 trial, but missed its primary statistical endpoint — setting up a tricky regulatory path despite strong efficacy and safety signals.
—Meghana Keshavan, STAT, 23 Mar. 2026
See All Example Sentences for efficacy
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Mary Kekatos, ABC News, 24 Mar. 2026
Finding passions, hobbies, community and values outside of family can help build self-worth and self-efficacy, Cole said.
—Rachel Hale, USA Today, 24 Mar. 2026
Still, the company said the shot reduced the rate of infection by more than 70% in people who received the vaccine versus placebo, efficacy the company thinks is strong enough to take to regulators.
—Angelica Peebles, CNBC, 23 Mar. 2026
Pfizer and Valneva’s Lyme disease vaccine cut cases by about 73% in a Phase 3 trial, but missed its primary statistical endpoint — setting up a tricky regulatory path despite strong efficacy and safety signals.
—Meghana Keshavan, STAT, 23 Mar. 2026
See All Example Sentences for efficacy
Cite this Entry
“Efficacy.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/efficacy. Accessed 31 Mar. 2026.
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