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EXPLAINER: Efficacy vs effectiveness of vaccines

Ivan Diaz on Unsplash

You’ve heard people use them as if they’re the same thing, but they’re not.

Efficacy and effectiveness of COVID-19 vaccines have become points of considerable global concern and discussion over recent months, and some commentators have been using them as if they’re talking about the same thing.

But there’s a difference.

Efficacy is …

Basically how a vaccine performs under trial conditions. 

Gavi, the Vaccine Alliance, (previously known as the Global Alliance for Vaccines and Immunization) explains it this way in a November 2020 article:

“Efficacy is the degree to which a vaccine prevents disease, and possibly also transmission, under ideal and controlled circumstances.”

Or as Live Science explains in a great article from February 2021 about the efficacy of the Pfizer vaccine:

“What the 95% actually means is that vaccinated people had a 95% lower risk of getting COVID-19 compared with the control group participants, who weren’t vaccinated. In other words, vaccinated people in the Pfizer clinical trial were 20 times less likely than the control group to get COVID-19.”

Effectiveness is … 

In short, how a vaccine performs in real-world conditions. 

Also in a February 2021 explainer, the Washington Post notes of vaccine effectiveness:

“Although it is possible the shots will be as effective in the real world as they were in the trials, “the efficacy numbers usually dip” when vaccines or other medical interventions are released into the general population, says William Schaffner, medical director of the National Foundation for Infectious Diseases.”

This “dip” could be due to a variety of factors.

Here’s another interesting article to read on this topic.

So, in summary, using the two terms as if they mean the same thing is being misleading. In times of an emergency, such as the COVID-19 health emergency, it is important to be accurate and to use the correct words and phrasing when communicating on COVID-19 pandemic health topics. 

28th March 2021

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