The risk of viral spread at a rally like the one President Trump has planned for next weekend in Oklahoma is much higher, the experts said, because the rally will be indoors in a large arena and there will no requirement that attendees wear masks.

“It’s a perfect storm setup: the idea of tons of people, where one sick person can have an impact of generating secondary cases on this immense level, where it’s indoors, where there’s no ventilation,” Dr. Bhadelia said. “I would move it to the outdoors, I would reduce the number of people, I would introduce social distancing, and I would require everybody to wear a mask.”