A high-efficiency particulate air (HEPA) filter can remove 99.97% - 99.99% of airborne particles that are equal to, smaller or larger than 0.3 microns in size (EPA). The efficiency is rated based on the worst level possible, so think about it as 99.97% or BETTER.
HEPA filters are tested using air particles that are 0.3 micron size as those are the most difficult size for a HEPA filter to catch. HEPA filters are actually more efficient at capturing smaller-sized particles, like those the size of viruses (which on average are 0.1 microns), due to how these smaller size particles behave (see NASA research). This is based on MERV (Minimum Efficiency Reporting Values) ratings as determined by the American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE).