It might shorten the time they survive. But no one apparently has built up a virus half-live vs UV intensity vs UV wavelength 3D chart yet.

https://sites.nationalacademies.org/BasedOnScience/covid-19-does-ultraviolet-light-kill-the-coronavirus/index.htm

UV-C definitely kills it, but the air is pretty opaque to these wavelengths. It also burns the crap out of your skin. UV-C, from my experiences with such lamps, will produce quite a bit of ozone, which you can easily smell. It will burn you pretty badly even with rather short term exposure from an 18 watt mercury vapor type emitter, within a range of about 3 to 5 feet, but beyond about 25 feet seems to not burn as much.

I'm not sure exactly how far the 245 nm emission line really travels through air, as all the fluorescent material I had also reacts strongly to the 365 nm UV-A emission line as well, which penetrates a lot further in air. The 365 nm line is used in mineraological and "poster lamps" and probably doesn't have any significant virus killing possibility.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3292282/

http://gsp.humboldt.edu/OLM/Courses/GSP_216_Online/lesson2-1/atmosphere.html

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Atmospheric absorption percentages throughout the electromagnetic spectrum. Image Credit: NASA

I suspect we will.  But the city the stadium is in is prepared for large rowdy crowds, just not sure the are prepared for angry ARMED crowds since Arizona is a open carry state with a glitch in the law that allows teens to carry rifles and anyone to carry a semi auto.

We have a UV light system on our HVAC system we just installed in January.

Lol!  Innovative approach.  Naked means no weapons!

The GOP has already contacted the campaign to get it to Phoenix inspite of the fact he has not paid for the rallies he held here or in Tucson.

A UV light system in an HVAC is the kind of game changer to effectively get ahead of infectious disease outbreaks. They make a system (kind of expensive) that also uses counterflow heat exchangers to bake the air to about 350 degrees to kill just about everything, then cools it back down to something normal, and recover most of the energy needed to do the heat up and cool down.

Essentially there are a pair of ducts with an oven at the far end. Pathogen laden air is drawn in one end. There is an accordian of copper or maybe copper clad aluminum, separating the two air streams. It has maybe half an acre of surface area so the hot air coming out of the oven, transfers its heat to the cool air going into it, while cooling off the super-hot air to near room temperature by the time it gets to the discharge point. Only a little refrigeration is needed to chill that down the last few degrees, and a little supplemental heat to pre-heat the intake air on the way to the oven. Sometimes they can use some of the a/c condenser core inside the intake to pre-heat for the oven, and make the process more energy efficient.

http://www.thermex.co.uk/news/blog/605-why-counter-flow-heat-exchangers-are-more-efficient

https://www.explainthatstuff.com/how-heat-exchangers-work.html

... A heat exchanger is a device that allows heat from a fluid (a liquid or a gas) to pass to a second fluid (another liquid or gas) without the two fluids having to mix together or come into direct contact. ...

... A recuperator is typically used to capture heat that would otherwise be lost, for example, as stuffy air is ventilated from a building: cold, incoming fluid is channeled in the opposite direction to warm, outgoing fluid to minimize the heat loss. The two fluids flow through separate channels, remain separate, and do not mix. Since incoming and outgoing fluids move in opposite directions, recuperators are examples of counterflow heat exchangers. The heat exchanger in a heat-recovery ventilation (HRV) system is an everyday example of a recuperator. ...

Phoenix in August? In the midst (more or less) of a pandemic?

Hmmm.