The U.S. is projected to see nearly 100,000 more COVID-19 deaths between now and December 1, according to the nation’s most closely watched forecasting model. But health experts say that toll could be cut in half if nearly everyone wore a mask in public spaces.

In other words, what the coronavirus has in store this fall depends on human behavior.

“Behavior is really going to determine if, when and how sustainably the current wave subsides,” said Lauren Ancel Meyers, director of the University of Texas COVID-19 Modeling Consortium. “We cannot stop Delta in its tracks, but we can change our behavior overnight.”

That means doubling down again on masks, limiting social gatherings, staying home when sick and getting vaccinated. “Those things are within our control,” Meyers said.