In circulation

“In circulation” is a phrase used to describe something that is currently being used or distributed within a group. It is often used in contexts such as money, publications, or items that are actively being passed around or available.

For example:

• “There are approximately 288 billion American pennies in circulation.” This means that there are that many pennies out in the population (including the ones in between our couch cushions!).
• If you say, “The library has several copies of that book in circulation,” it means the library has several copies of the book that are available for people to borrow.

It’s a way of indicating that something is actively being used or distributed within a particular system.

This can also be used for diseases. During the COVID-19 pandemic, we used “in circulation” this way: Moderna is already developing booster shots that target the Omicron variant that’s currently “in circulation,” or that’s currently passing from person to person.