What is the difference between efficacy and effectiveness?

The two terms used to describe how well a drug or vaccine works are often used interchangeably, but they are not actually the same thing – here’s why.

  • 18 November 2020
  • 3 min read
  • by Priya Joi
Gavi/2020/Maya Hautefeuille
Gavi/2020/Maya Hautefeuille


Efficacy is the degree to which a vaccine prevents disease, and possibly also transmission, under ideal and controlled circumstances – comparing a vaccinated group with a placebo group. Effectiveness meanwhile refers to how well it performs in the real world. Although a vaccine that has high efficacy – such as Moderna’s COVID-19 vaccine with 94.5% efficacy and Pfizer’s with 90% efficacy – would be expected to be highly effective in the real world, it is unlikely to translate into the same effectiveness in practice.

What is efficacy?

A vaccine with an efficacy of 90% in a trial, for instance, means there was a 90% reduction in cases of disease in the vaccinated group compared to the unvaccinated (or placebo) group. But efficacy in laboratory conditions does not always translate to effectiveness, and so 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 – people are often not included in trials if they have underlying health issues, or are taking medication – and side effects are closely monitored.

Moreover, 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 they may also need to receive the vaccine when one is ready.

How do we measure effectiveness?

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 administered under everyday conditions, can reduce how effective the vaccine is at preventing disease.

Once the efficacy of a vaccine has been determined, measuring its effectiveness is critical to ensuring uptake of the vaccine and to understand how to develop better vaccines. Surveillance data is vital to understanding effectiveness, as is immunisation data – capturing data, for example, on when people get the vaccine and what proportion of the population in a given country is covered.

Effectiveness of a vaccine is measured in what epidemiologists call observational studies because participants are not randomly assigned to a 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 an exceptionally high effectiveness to be useful, for example the influenza vaccine is 40-60% effective yet saves thousands of lives every year.