Some furnace filters claim to filter viruses(e.g. filtrete 1500 and higher from 3M) and seem to still be available in stores/amazon. These should be repurposed for higher priority users. (Preferably with a better seal than this.)
From filtering point of view viruses are dust particles of roughly 100 nanometers in diameter and aerosols. Lots of filters are designed to filter this stuff, pretty much all ultrafine particle class filters.