What is the difference between efficacy and effectiveness? In day to day language, we exchange the two as they are relatively similar synonyms.

However, in the context of understanding vaccines (and any other kind of treatment, therapy, etc.), they have important nuances!

In the context of vaccines:

Efficacy refers to how well it prevents disease in controlled environments such as a lab. These are the numbers we’re seeing in the news cycle, as we have yet to learn about effectiveness.

Effectiveness refers to how well this vaccine may work in the real world, which can be different because reality is messy, complex, and filled with random occurrences and people who behave in ways we can never truly predict.