While air filters are not enough in themselves to prevent Covid 19, they are an important part of the range of activities that you should undertake to prevent the spread, including masking and social distancing. For removing viruses from the air, an air cleaner needs to be able to remove particles that are in the range of 0.1-1 um. This can be read in many ways: for example, in some cases you might see the label 99.9% particles removed with particles as small as 0.3um. I have mentioned above the CADR measurement and how it varies with room size. Therefore, you need to be careful when choosing your air filters and purifiers and buy the one with the right CADR based on your room size. Units that mention specifically that they are good for wildfire smoke, or have HEPA filters are generally a good option in this respect.