It has already been tried out at a country level in Japan. Ventilation, and HEPA filters.
But what you mean here is “Japan have done things with ventilation”, rather than “Japan has put HEPA filters on all its buses”, presumably?
Again, I am NOT saying ventilation is pointless. I am saying that the cost of putting in ventilation in a particular way in a particular place might outweigh the benefits.
So like, maybe it doesn’t work so well in buses because it costs a fortune, and it means bus companies pass the prices on to customers and close down routes, and people aren’t usually on buses for long enough for it to make much difference. And maybe it works really well in offices, where people are in one place for longer. But you can’t put it in offices because you’ve just wasted £119 million putting HEPA filters on buses, and now you have no money left for the offices, ooops.
Or maybe it works sort of well in classrooms but not hugely well, and it costs a fortune. And there’s another ventilation system we could do instead which works a bit less well for the virus, but works much much better at reducing air pollution, and is cheaper. So maybe we should do that instead? Except we can’t, because we’ve already spent the money on the first system before looking at how well it works.
It would be irresponsible to spend billions of pounds based on nothing more than “Japan does ventilation-y stuff and I bet that’s what’s behind their low cases, so it must be worth it.”