By definition, HEPA filters capture 99.97% of particles that are 0.3 microns in size. This is great for common household contaminants like dust and bacteria, which can measure as little as 0.5 and 0.3 microns respectively.
Unfortunately, the COVID-19 virus is approximately 0.125 microns in size, which exceeds HEPA filter requirements. Though HEPA filters do help to reduce levels of airborne impurity, such filtration alone is ineffective as protection against the COVID-19 virus.
Although HEPA filters are unable to deal with COVID-19 directly, they do greatly improve the overall air quality, which will have significant health benefits. Consequently, HEPA filters can be considered a helpful measure against airborne threats more generally.