The U.S. is projected to see nearly 100,000 more COVID-19 deaths between now and Dec. 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.

The U.S. is in the grip of a fourth wave of infection this summer, powered by the highly contagious delta variant, which has sent cases, hospitalizations and deaths soaring again, swamped medical centers, burned out nurses and erased months of progress against the virus.

Deaths are running at over 1,100 a day on average, turning the clock back to mid-March. One influential model, from the University of Washington, projects an additional 98,000 Americans will die by the start of December, for an overall death toll of nearly 730,000.

The projection says deaths will rise to nearly 1,400 a day by mid-September, then decline slowly.

But the model also says many of those deaths can be averted if Americans change their ways.

“We can save 50,000 lives simply by wearing masks. That’s how important behaviors are,” said Ali Mokdad, a professor of health metrics sciences at the University of Washington in Seattle who is involved in the making of the projections.

Already there are signs that Americans are taking the threat more seriously.

Amid the alarm over the delta variant in the past several weeks, the slump in demand for COVID-19 shots reversed course. The number of vaccinations dispensed per day has climbed around 80% over the past month to an average of about 900,000.

White House COVID-19 coordinator Jeff Zients said Tuesday that in Alabama, Arkansas, Louisiana and Mississippi, “more people got their first shots in the past month than in the prior two months combined.”

Also, millions of students are being required to wear masks. A growing number of employers are demanding their workers get the vaccine after the federal government gave Pfizer's shot full approval earlier this week. And cities like New York and New Orleans are insisting people get vaccinated if they want to eat at a restaurant.

Half of American workers are in favor of vaccine requirements at their workplaces, according to a new poll from The Associated Press-NORC Center for Public Affairs Research.

Early signs suggest behavior changes may already be flattening the curve in a few places where the virus raged this summer.

An Associated Press analysis shows the rate of new cases is slowing in Mississippi, Florida, Louisiana and Arkansas, some of the same states where first shots are on the rise. In Florida, pleas from hospitals and a furor over masks in schools may have nudged some to take more precautions.

However, the troubling trends persist in Georgia, Kentucky, South Carolina, Tennessee, West Virginia and Wyoming, where new infections continue to rise steadily.

Mokdad said he is frustrated that Americans "aren’t doing what it takes to control this virus.”

“I don’t get it,” he said. “We have a fire and nobody wants to deploy a firetruck.”

One explanation: The good news in the spring — vaccinations rising, cases declining — gave people a glimpse of the way things used to be, said Elizabeth Stuart of Johns Hopkins Bloomberg School of Public Health, and that made it tough for them to resume the precautions they thought they left behind.

“We don’t need to fully hunker down,” she said, “but we can make some choices that reduce risk.”

Even vaccinated people should stay vigilant, said Dr. Gaby Sauza, 30, of Seattle, who was inoculated over the winter but tested positive for COVID-19 along with other guests days after an Aug. 14 Vermont wedding, even though the festivities were mostly outdoors and those attending had to submit photos of their vaccination cards.

“In retrospect, absolutely, I do wish I had worn a mask,” she said.

Sauza, a resident in pediatrics, will miss two weeks of hospital work and has wrestled with guilt over burdening her colleagues. She credits the vaccine with keeping her infection manageable, though she suffered several days of body aches, fevers, night sweats, fatigue, coughing and chest pain.

“If we behave, we can contain this virus. If we don’t behave, this virus is waiting for us,” Mokdad said. “It’s going to find the weak among us.”

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The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Department of Science Education. The AP is solely responsible for all content.