HEPA filters are actively tested for filtration down to 0.3 Microns. This does not mean they cannot trap smaller particles. In fact, they are found to do a better job on even smaller. In 2016 NASA did a study
on submicron particles (less than 1 micron in size). They found that HEPA filters removed near 100% of nanoparticulate matter. The graph to the right is from page 7 of the study and shows a high capture rate of particles that include those the size COVID-19 virus. A further affirmation of HEPA Filtrations value can be sited in ASHRAE Position Paper
from April 14, 2020. Here ASHRAE calls for Portable and local HEPA filtration in all medical and non-medical buildings.