“Second waves are inevitable in pandemics when you don’t have a vaccine,” said Carlos Del Rio, head of the global-health department at Emory University, who chaired the panel that guided the National Collegiate Athletic Association to shutting down sports this spring. “Any disease when you have an epidemic, when you loosen up prevention, you’ll have a second wave.”

The question of when American life gets back to normal is not whether there will be another spike in cases, hospitalizations and deaths. Del Rio says it’s this question: “How much of a second wave are you willing to tolerate?”…

For some scientists, it’s a probability problem. The prevalence of cases is unlikely to sink low enough by this fall to host a football game without the high risk of someone infected being in the crowd. The more people in the crowd, the greater the chance that at least one is infected, and the more people the infected will be in contact with. All it takes is one game to trigger a local outbreak.

“I think the risk of a second wave is a huge risk, and you’re playing with fire holding a football game with people in the stands,” said Carl Bergstrom, a biology professor at the University of Washington.