On public transport? Where people are getting on and off the train or bus every few minutes and you don't have a closed system?

I don't know about the specifics of UV air filters but I'm not sure they'd be an alternative in this particular scenario. They seem to be more useful in situations where the same people are in the same room for a prolonged period of time.

I'm all in favour of small, incremental improvements. Those can be a huge contributing factor, so we don't have to wield the blunt instrument that is lockdowns anymore.

Using air filters probably is more of such an additional improvement, rather than a complete solution in its own right.