Sorry to burst your collective bubbles, but cabin air filters won't filter down to 0.1 micron (which is the particle size of the virus) they are there to prevent dust and lint build up, or are there to protect sensitive equipment (like outflow valves reference lines)
Even if all the individual 'punkah vents' are closed, then the relieving valves (nothing fancy, just spring loaded flaps) in the ductwork ( behind overhead bins usually) will open , and that air will come into cabin anyway.
So, open or closed, will make no difference to air coming in to cabin, but closed will reduce chance of a direct blast to the face.
Ttfn