My sorority had a similar problem when I was in college. We had a measles outbreak on campus and an epidemic in the local schools in town. We didn't have to cancel our sorority semi-formal, but we definitely had to make changes. We all had to carry proof of our vaccinations as did our dates. We wound up moving the location to outside of town. Luckily, our original place was sympathetic to the situation and didn't penalize us and returned our money.
I think that you should contact the original place and ask them very nicely to return your deposit in full. This change is not your fault. There is a medical emergency and potential epidemic. You can also add how you would love to utilize their facility in the future and would encourage other groups to do the same. Refunding the deposit would show that they also want to build a long term relationship with your sorority and the university. It might work.
Did the university tell you to cancel the event in Toronto or just strongly encourage it? If it was mandatory then the university should re-emburse you or accomodate you in another way. Maybe they can talk to tthe facility and explain the situation.
I am not sure what you can do to make the semi-formal more fun. Sisters, alumnae, decorations, good food, music, and dancing is really all that a semi-formal is. The location is irrelevant. It is supposed to be a celebration of a year's work and a way to say goodbye to the seniors and welcome back the alumnae. If that's not enough, I don't know what it. Maybe if you get some of the movers and shakers to attend, others will follow. Good luck! Stay healthy!
