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As government officials across the nation wrestle with choices over shutdown policies in a pandemic that has hit some places harder than others, a new study sheds light on how much of a difference stay-at-home orders make on lives saved and infections.

The study, by Stanford economist Matthew Gentzkow and his colleagues, finds that social distancing behaviors clearly matter in terms of reducing mortality and infection rates, but that most social distancing in the early pandemic was independent of whether or not areas had lockdown mandates in place.

“The course of the pandemic has varied dramatically. Some places have been harder hit than others; the waves of coronavirus cases have happened in different places at different times,” said Gentzkow, a senior fellow at the Stanford Institute for Economic Policy Research (SIEPR). “And as we are making choices about how to address the pandemic, it’s incredibly important to understand where that variation is coming from.”

The study, detailed in a working paper recently posted by the National Bureau of Economics, could provide some clarity in a public discourse laced with finger-pointing and unknowns.

“There’s a lot of loose jumping to conclusions, trying to explain why cases are spiking in some places, and if it’s because places were slower to lock down, or if it’s because people in those places weren’t wearing masks,” Gentzkow said. “But it’s good to have evidence to what extent those factors are important drivers.”

While the study did not examine mask-wearing, the researchers used a combination of real-world data from February through April along with a commonly used epidemiological modeling framework to tease out the effects of shutdown policies across the nation. Social distancing behaviors were measured by anonymized GPS data to capture overall movement patterns.

The researchers’ calculations translate to sobering estimates of what happened, and what could have been.

Specifically, county lockdown policies — including stay-at-home and business closure restrictions — reduced disease transmission rates by 9 to 14 percent between early March and mid-April, the study found.

“If there were no lockdowns at all, we show in the paper that there would have been twice as many coronavirus cases by the end of April,” Gentzkow said. “But the magnitude of their effect is relatively modest compared to the social distancing that people are doing on their own, independently of policy.”

People were social distancing substantially even before stay-at-home orders came down, and their mobility levels remained significantly depressed even after many such policies were lifted, the study found.

The researchers estimated that if all U.S. counties had adopted a stay-at-home policy on March 17 — which is when the San Francisco Bay Area became the first in the nation to issue a quarantine order — then there would have been a 20 percent reduction in, or 154,000 fewer coronavirus cases by April 30.

But the reduction would have been even larger if quarantine policies were accompanied by the same high level of social distancing behaviors exhibited by the San Francisco region: a decrease of 62 percent, or 494,000 fewer cases in that period.

Based on an observed case mortality rate of 3 percent, shutdown policies alone saved an estimated 14,900 lives across the nation. And if all counties were to have socially distanced like the San Francisco region, then an additional 10,500 lives would been saved.

At the same time, other fixed characteristics play a huge role, the researchers say.

To better understand why some U.S. cities were harder hit by COVID-19 than others during that period, the researchers analyzed a variety of potential factors — from lockdown policies and social distancing to commute patterns, political leanings and international flight connections. They found that at least during the early stage of the outbreak, population and density stood out above everything else as major drivers for virus transmission.

The study also examined some of the economic impacts of lockdown policies, and found that they reduced consumer spending by 7 percent and small business employment by 12 percent.

In addition to Gentzkow, the co-authors of the study were Hunt Allcott of Microsoft Research, Benny Goldman of Harvard, and three Stanford PhD candidates — Levi Boxell, Jacob Conway, and Billy Ferguson.

While the researchers — themselves in quarantine — sorted the data for answers, debates around them blared.

“Often, the narrative in the media is, ‘Oh! Clearly, these areas are being hit much worse and it's all driven by their differences in policy.’ I don't think that's true. Or, there would be statements like, ‘Oh! These lockdowns are doing nothing.’ That's also not true,” Conway said.

“We found the policies have modest effects,” said Conway. “They’re important, but they're not perfect. So, the answer is somewhere in between.”

The reality behind the numbers are stark.

“In terms of the sheer number of lives saved, the fact that we could save any lives — these are incredibly important policy decisions that affect so many people,” Conway said.