We've converted all filters to MERV 13 rather than the MERV 8 we normally use. Higher numbers are better and filter more.
In addition to wearing face masks and staying at least six feet away from other people, good air circulation inside buildings will reduce the spread of COVID-19 in aerosols. During Spring, Summer, and Fall, we circulated outdoor air through the store continually. When the weather cooled and we needed to be able to close the doors, we converted to MERV 13 air filters, and set the air to recirculate continually.
Ventilation with clean outdoor air is healthier because it removes viruses and other particles, and also removes gasses like the carbon dioxide that everyone makes when they breathe out. However, outdoor air is hard to heat or cool so most ventilation systems use at least some recirculated air.