As infections four years ago from a novel virus rapidly circulated across the country and states began enforcing shutdowns to prevent its spread, it was then unknown how much changing American’s behaviors could thwart the disease.

A new study suggests social distancing helped prevent 800,000 COVID-19 deaths before a vaccine could be developed, research by UCLA and the University of Colorado Boulder shows.

Social distancing included avoiding crowds, wearing facemasks and keeping at least six feet away from others.

Stephen Kissler, a mathematical epidemiologist at CU Boulder and co-author of the study, said these behavior changes were “a powerful force for slowing the spread.”

“Every future pandemic will now be seen through the lens of COVID,” Kissler told The Denver Gazette.

People all over the globe are now much more aware of best practices during a pandemic, which include good hygiene — washing hands, covering coughs — and social distancing.

To date, COVID-19 has killed roughly 1.2 million Americans, including more than 15,500 Coloradans and roughly 1,600 Denver residents. COVID-19 wasn’t the deadliest pandemic recorded. That distinction goes to the Black Death, which killed as many as 200 million people globally. Nor is COVID-19 the deadliest influenza pandemic. That would be the Spanish flu of 1918 — the first global pandemic in the era of mass society — killed as many as 100 million worldwide.

Since its introduction five years ago, COVID-19 has killed 7 million people the world over.

In less than a year — a breakneck speed for scientists — researchers developed several vaccines that authorities and supporters have said changed the course of the disease.

Both the federal and local governments enforced vaccine mandates at the height of the pandemic. The Biden administration mandated large companies to require their workers be vaccinated or tested weekly. In Colorado, the Polis administration required licensed facilities to vaccinate their workforce against the virus.

Kissler and Andrew Atkeson, a UCLA economics professor, wanted to know how many deaths were prevented by behavioral interventions and vaccination.

Using computer models, they simulated — mathematically — scenarios to answer questions for the study, such as: How many Americans would have died without facemasks and social distancing? And, how many would have died without vaccines? The study found an inextricable link. “Without vaccines, behavior alone would have postponed infections, but in the end, nearly everyone would have been infected and subject to a high infection fatality rate from that first infection,” Kissler and Atkeson wrote.

“Without a behavioral response, vaccines would have come too late to save lives.”

For the study, researchers analyzed national serology data that examined the antibodies in blood samples and mortality data from the Centers for Disease Control and estimated how many Americans had been infected with or vaccinated against COVID-19 at various points over a four-year-period from 2020 to 2024.

Among the study’s findings:

• An estimated 68% of Americans were vaccinated against COVID-19 before becoming infected.

• 1.98 million Americans would have died without the vaccines or behavior changes related to social distancing.

• Likely 273,000 preventable deaths occurred because of declining vaccinations in late 2021.

Without social distancing and vaccines, the researchers estimated 1.98 million Americans would have died from COVID-19.

As the shutdowns wore on and adversely affected schools and commerce with unemployment numbers soaring, the American public began to sour on the pandemic strategies, which included shutting down schools, businesses, events, and — in some parts of the country — places of worship. Meanwhile, people who refused the vaccine mandates lost their jobs.

Notably, schools reported a precipitous dive in student performance after administrators closed buildings and shifted into remote learning. The effect of remote learning still reverberates to this day with school districts struggling to raise the academic performance of their students to pre-pandemic levels.

“My concern is that the next pandemic will be deadlier, but people will ignore it, because they will say, ‘Oh, we overdid it during COVID,’ ” Atkeson said in a statement.