Question about Xiaomi Mi Air Purifier 3H

I agree that it's best to ask the manufacturer. HEPA filters in principle filter out 99.95% of 0.3 micron particles. This is equivalent in efficiency to an FFP3 dust mask, which (to the best of my knowledge) has the best commercially available small-particle filtration efficiency for respiratory masks. Virus particles tend to hitch a ride on aerosol particles, and there are many recent studies suggesting that it's these aerosol particles that are responsible for much of the virus transmission (they hang around in the air for a while). So the more of those you filter out, the better your protection from the virus should be. But like Davidbag says, actual virus particles are much smaller than this, so if there are too many virus particles that are themselves airborne, these will likely not get caught by either a HEPA filter or FFP3 mask.

I think none of the HEPA filters are effective with any sort of virus, since the virus particle are smaller that dust particles...
but that you should ask to the manufacturer!

The problem with these masks is that they only filter air taken in for the wearer, they have no outbound filtration and therefore may pose a risk to others if you are Covid positive.

I would certainly not recommend wearing masks such as these around vulnerable people if you are not testing daily to ensure you are virus free.