Even a cheap HEPA filter is good enough to filter COVID.
But, all contaminated air has to actually go through the filter.
This means you need to do several full air exchanges in the required space per hour really if you want to keep people from getting sick. Like a laboratory clean room would do.
And setting up that level of HEPA filtration, is fucking expensive.
I built a very small flowhood for mycology. The filter was $200. The fan another $80. And I’m only keeping a one x two space clean. I imagine it’s just not economical to really apply this technology to schools etc at this point in time.