This cross-sectional study  found an increase in estimated rates of COVID-19 cases in the border counties in Iowa (no stay-at-home order) compared with the border counties in Illinois (implemented a stay-at-home order). In short, the results suggest that issuing a stay-at-home order in Iowa may have helped limit the spread of COVID-19 cases in that state.

• The study compared daily changes in COVID-19 cases per 10 000 residents in 8 Iowa counties bordering Illinois with those in the 7 Illinois counties bordering Iowa before and after Illinois issued a stay-at-home order on March 21, 2020
• Trends of cumulative COVID-19 cases per 10 000 residents for the Iowa and Illinois border counties were comparable before the Illinois stay-at-home order, which went into effect at 5:00 pm on March 21 (March 15 to March 21: 0.024 per 10 000 residents vs 0.026 per 10 000 residents)
• After that, cases increased more quickly in Iowa and more slowly in Illinois.
• Within 10, 20, and 30 days after the enactment of the stay-at-home order in Illinois, the difference in cases was −0.51 per 10 000 residents (SE, 0.09; 95% CI, −0.69 to −0.32; P < .001), −1.15 per 10 000 residents (SE, 0.49; 95% CI, −2.12 to −0.18; P = .02), and −4.71 per 10 000 residents (SE, 1.99; 95% CI, −8.64 to −0.78; P = .02), respectively.