Slyguy,
UV isn't used because typically the air velocities in the air con systems on aircraft are too high (dwell time is not sufficient) for the UV as efficiently deactivate the bacteria/virus as the currently installed HEPA filters is at blocking its passage. There are some prototype UV systems knocking about, but they've not gone further - apart from dwell time there's pressure drop, maintenance cost, reliability and weight that all play against UV on aircraft.
HEPA filters are installed on Airbus - don't know about the smaller regionals. Numerous studies have shown the efficiency of HEPA. The main transmission method on aircraft is person to person (and that's before you look at the whole travel experience, queuing at check-in and security, mixing in the terminal with people from goodness-knows-where, etc.)
Hope that satisfies your curiosity
Edited to note that the filter suppliers quote 99.97% efficiency for HEPA filters.