In medicine, effectiveness is not the same as efficacy. Here’s what you need to know

Have you ever heard the words efficacy and effectiveness being used interchangeably? You probably have used them as synonyms at times, especially in describing COVID-19 vaccines. 

Well, this article is here to first inform you of their difference, with a focus on vaccine efficacy and effectiveness which has been wrongly used and has thus led to one form of misinformation or another.

Let’s start with the term public health impact. This is usually measured to place the association between an exposure and an outcome into a meaningful public health context. Measures of public health impact reflect the burden that exposure contributes to the frequency of disease in the population and two methods are often used; the attributable proportion and efficacy or effectiveness. 

The term efficacy and effectiveness have been used interchangeably when it comes to vaccines and more often with the rollout of the COVID-19 vaccine.

The two terms are used to describe how well a drug or vaccine works; however, they are not actually the same thing.


Efficacy is the degree to which a vaccine prevents disease and transmission during a trial, that is, under ideal and controlled circumstances in which a vaccinated group and an unvaccinated group, otherwise referred to as a placebo group, are compared in terms of effect on the two groups.

A vaccine with an efficacy of 90% in a trial, for instance, means there was a 90% reduction in infection in the vaccinated group compared to the unvaccinated or placebo group.


Effectiveness on the other hand refers to how well the trial drug performs in the real world when administered.

According to a study by D S Fedson, an efficacy study asks the question, “Does the vaccine work?” while an effectiveness study asks the question “Does vaccination help people?”

Although a vaccine that has a high efficacy rate would be expected to be highly effective in the real world, but this is unlikely to translate into the same effectiveness in practice.

Having looked at both meanings, it is important to note that efficacy in laboratory conditions does not always translate to effectiveness, thus indicating that an efficacy trial can overestimate a vaccine’s impact in practice.

In clinical trials, the conditions under which a participant is taking a vaccine are carefully designed. This means people are often not included in trials if they have underlying health issues or are taking medication that may have side effects.

Also, participants in the trial represent a subsection of the full age range of a population. For example, not many COVID-19 vaccine trials have included young children, even though there will be a need to vaccinate them soon.

When a vaccine is given to the population, factors, such as the medication people are taking, underlying chronic illnesses, age, and how the vaccine is stored and under what conditions the vaccine is administered, can reduce how effective the vaccine is at preventing disease.

How then do we measure effectiveness?

Once the efficacy of a vaccine is determined, measuring its effectiveness is critical to ensuring uptake of the vaccine and to understand how to develop better vaccines. 

Therefore, surveillance data is vital to understanding effectiveness, as is immunisation data or capturing data.

This data will cover when people get the vaccine and what proportion of the population in a given country is covered.

The effectiveness of a vaccine is measured in what epidemiologists call observational studies because participants are not randomly assigned to treatment versus a placebo group. For example, case-control studies assess effectiveness by comparing the vaccination status of individuals who develop the disease (cases) with a group of individuals without the disease (controls) who are also representative of the population from which the cases arise. If the vaccine is effective, the cases are more likely to be the unvaccinated individuals.

Vaccines do not always need to have exceptionally high effectiveness to be useful, for example, the influenza vaccine is 40-60% effective and yet saves thousands of lives every year.

What is the situation with available COVID-19 vaccines?

There are approximately 96 COVID-19 vaccines at various stages of clinical development. Presently, interim results of four studies published in scientific journals on the Pfizer, Moderna, AstraZeneca–Oxford and the Gamaleya vaccine exist and only three studies go through the US Food and Drug Administration (FDA) briefing documents on Pfizer, Moderna and Johnson & Johnson.

The efficacy percentage (i.e relative risk reductions) for the Pfizer vaccine is 95%, 94% for the Moderna, 90% for the Gamaleya, 67% for the Johnson and Johnson, and 67% for the AstraZeneca vaccines. 


Although attention has focused on vaccine efficacy, fully understanding the efficacy and effectiveness of vaccines is less straightforward than it might seem. Also, the way studies are conducted and results are presented with the use of only Relative Risk Reduction (RRR) omitting Absolute Risk Reduction (ARR), reporting bias is introduced and this affects the interpretation of vaccine efficacy.

It is, therefore, important to have the full picture of what the data actually show to ensure comparisons are based on the combined evidence that puts vaccine trial results in context when communicating about vaccine efficacy.

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