What’s the difference between efficacy and effectiveness in clinical trials? Posted on 21/12/2020

Problems with Scientific Research

Efficacy and effectiveness – what’s the difference?

Efficacy is the maximum performance of a treatment under ideal and controlled circumstances. This includes testing on healthy populations and within a time constraint i.e. test controls will be monitored for 14 or 30 days post administration.

To determine efficacy the outcome is plotted against dose in a graph, to give the dose – response curve. The increasing doses are displayed by the X axis (horizontal) and the half maximum and maximum responses are displayed by the Y axis (vertical). The highest point on the curve shows the MAXIMUM response (efficacy), not the average. 

Treatment must then be compared to a control – whether that be something as simple as a saline solution or in vaccine trials this can be another vaccine.

These studies are often double blinded – meaning both the participants and the researches don’t know which they received until the end of the trial.


Is the performance under real world conditions.

To determine effectiveness, monitoring of treatment does not stop after they are approved for use.

When the treatment is deployed, data will continue to be collected to study how well it works over the course of a few years.

Researchers will be looking at how well it works in different age groups, different ethnic groups, duration of protection and effectiveness against evolving strains, the balance of benefit vs harm, cost and longer term complications. This data collection can take several years for data to be reported adequately across different groups and the original efficacy rate is adjusted.   

As you can see, these terms cannot be used interchangeably. If a treatment has a 90% efficacy rate, please note that it’s the maximum effect recorded in a controlled environment on  a certain group of people, over the short-term.  

Effectiveness is what they’re setting out to determine over the course of the next few years. This will often adjust the original efficacy rate.

Be mindful when engaging in said conversations.

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