Iowa did not issue a state-wide stay at home order. The neighboring state of Illinois did. The counties at the border of the two states, where life is similar in many ways, found themselves at divergence. That difference in policy seemed to influence the number of COVID-19 cases.

The stay-at-home order in Illinois was issued on March 21, while Iowa was among only a handful of states that did not introduce any such order. Rates of COVID-19 had been similar across the Iowa and Illinois counties before March 21 — and for about a week afterward, they remained similar.

But the differences started in late April. After that, cases increased more quickly in Iowa and more slowly in Illinois.

“It’s early descriptive evidence that suggests that more restrictive measures may be related to slower growth of COVID rates,” says George Wehby, a study author and a professor of health management and policy at the University of Iowa.

Researchers merely carried out an observational study — they did not establish a cause-effect relationship, they merely observed the correlation between this policy and the number of COVID-19 cases.

There could be other aspects at play.

The total populations were 462,445 in the Iowa border counties and 272,385 in the Illinois border counties. Population density was higher in the Iowa counties (114.2 people per square mile) than in the Illinois counties (78.2 people per square mile), and this could be an alternative explanation. Other factors (which are not mutually exclusive) might also be at play, researchers add.

“There could be a messaging effect,” Wehby says. “We don’t know how these shelter-in-places are affecting people’s behavior when they’re outside … maybe there is some greater alertness (around) individual behaviors, like staying away from people more than 6 feet, even though you’re outside.”

However, researchers also note that Illinois also had a greater increase in tests per 10,000 residents after the stay-at-home order, suggesting that this isn’t a testing issue. In other words, bordering counties in Illinois really seemed to have fewer cases than the ones in Iowa, which correlates well with a stay-at-home order, but it’s still a correlation-not-causation type of study, so we can’t say for sure whether the stay-at-home order made the difference.

However, the circumstantial evidence seems to lend additional support to the policy making a difference.

A separate analysis published this week in Health Affairs indicates there may have been 10 million more COVID-19 cases in the U.S. by late April without government-imposed shelter-in-place orders.

At the time of this writing, the US, a country with approximately 4.25% of the world’s population, has some 30% of the total COVID-19 cases in the world.