“Effectiveness” vs. “Efficacy” vs. “Efficiency”: When To Use Each Word For The Best Results

Countries around the world are doing what they can to distribute the COVID-19 vaccine to as many people as possible. As we wait and hope the vaccine will finally free us from the virus’s clutches, the words are flying fast and furious; everyone from Dr. Fauci on down is suddenly concerned with terms like vaccine efficacy and vaccine effectiveness. And who can blame us for this sudden interest?

Before we look at what these terms specifically mean in epidemiology, let’s take a look at what the terms efficacy and effectiveness mean in general.

What does effectiveness mean?

Effectiveness is the main noun form of the adjective effective, which means “adequate to accomplish a purpose; producing the intended or expected result.” (Another, less common noun form of effective is effectivity.)

So, if you are measuring something’s effectiveness, you are looking at how well it does whatever it is supposed to do. If a dish soap only kills a small amount of germs on dishes, for example, it has poor effectiveness.

The adjective effective comes from the noun effect. An effect is “something that is produced by an agency or cause; result; consequence.” The word effect is often confused with the word affect, which is often used as a verb to mean “to produce an effect or change.”

An effective way to address the confusion around affect and effect is to read this article on the proper way to use them. We’re sure you’ll be less confused as a result.

What does efficacy mean?

Efficacy is “capacity for producing a desired result or effect; effectiveness.” The adjective form efficacious means “capable of having the desired result or effect.”

You may have encountered the word efficacy used in the term self-efficacy, which refers to a person’s belief that they can accomplish what they set out to do. For instance, a student has high self-efficacy if they take on a challenging job, fresh out of college with little experience, convinced they’ll succeed.

Efficacy and effectiveness are close enough in meaning that they are often used interchangeably in general contexts. However, effectiveness is often more specifically used in the context of how well something accomplishes a task whereas efficacy conveys the extent to which something accomplishes its task at all. If a dish soap doesn’t kill any germs at all, for example, we would say it is not an efficacious germ-killer.

Careful: effective is not the same as efficient

Neither of these words should be confused with efficiency, which is “the state or quality of being efficient, or able to accomplish something with the least waste of time and effort; competency in performance.” You can think of efficiency as going a step further: it not only measures how well something does its job, but how quickly and/or cheaply it can do it.

Discerning the difference (and similarities!) between effective and efficacious can be tricky, especially when you consider related words like efficient and effectual. One effective way to understand these terms? Take a quick look at our Synonym Study of the word effective that separates out all of these very similar words. As always, you can thank the English language–and its roots in Latin—for making learning these definitions and connections so, er, fun?

Before we move on, it is important to point out a few reasons why these words are so confusing. The first is that they all, of course, have to do with getting stuff done. Second, all of these words look really similar to each other. There is actually a reason behind it: they all ultimately come from Latin verbs based on the root ex- (in this case meaning “thoroughly”) and the verb facere (“to do” or “to make”).

Go Behind The Words!

Get the strangest stories of your favorite words in your inbox.
  • This field is for validation purposes and should be left unchanged.

Vaccine efficacy vs. vaccine effectiveness

While efficacy and effectiveness may function as synonyms in general usage, they actually have a specific difference in epidemiology, especially when referring to vaccines.

Vaccine efficacy is a measure of how well a vaccine lowers the prevalence of a disease under ideal, controlled conditions. In other words, it is a measure of how well a vaccine prevents the spread of a virus when tested in a research laboratory or highly controlled study.

Vaccine effectiveness is a measure of how well a vaccine lowers the prevalence of a disease under “real world” conditions. In other words, it is the measure of how well a vaccine prevents the spread of the disease when actually used out in society.

Putting it all together, scientists are interested in the difference between these two measurements because it provides important information about the vaccine that may not be testable in a lab, such as if the vaccine is too expensive or if it is affected by a condition (e.g., a patient’s age) that the researchers may not have accounted for.

Scientists are now keeping a close eye on COVID-19 vaccine distribution and seeing how the vaccine effectiveness compares to the vaccine efficacy. Regardless of which word we use, one thing is for sure: we all want this pandemic to be over!

Build up your lexical defenses with this analysis on the differences between "vaccinate," "inoculate," and "immunize."