Notre Dame Withdraws From Hosting Presidential Debate Due To COVID-19

Notre Dame decided not to host the first 2020 presidential debate due to health precautions affecting its educational value. The event will now be co-hosted by Case Western Reserve University and the Cleveland Clinic, which will implement risk mitigation procedures. The University of Michigan also withdrew from hosting a debate due to COVID-19 concerns.

What factors would determine whether an audience would be present at the debate?

What concerns led the University of Michigan to withdraw from hosting the second presidential debate?