How to avoid getting COVID in an air-conditioned office building?

I work in an office in a building with central air. I leave the office door closed when I am inside, and no one else comes in. I don’t know much about the ventilation in the building, other than that it is a “normal US office building,” built sometime during the past 20 years, in the northern half of the US, during winter.

- Should I put something over my AC vent (i.e. a filter or something to block it entirely)? Why or why not?

- Are there any easy other actions I can take to reduce risk (other than wearing a mask or not going into the office)?

- Does the answer change based on how much the AC is pulling in air from outside the building versus just recirculating it throughout the building?