News Release

Mask mandates reduce COVID-19 transmission in rural US counties

Study shows that masks are effective in areas of low population density

Peer-Reviewed Publication

De Gruyter

Small, rural US counties that mandated mask wearing saw lower rates of COVID-19 transmission than counties that did not introduce a mandate, new research published in De Gruyter’s Journal of Osteopathic Medicine finds.

The study, Evaluating the Effectiveness of Countywide Mask Mandates at Reducing SARS-CoV-2 Infection in the United States”, also showed that introducing a mask mandate in a county significantly reduced infection rates compared to a control.

The research examined counties with populations between 40,000 and 105,000 in four randomly selected states - Missouri, Iowa, Tennessee and Florida - that did not have statewide mask mandates. In total, 38 counties were included, half of which had introduced mask mandates and were test counties, while the others had no mandates and were the controls.

The study determined the infection rates of each county for 30 days after mask mandates were passed. Test counties were found to have an average of 19.63 new COVID-19 infections per day, while the controls had an average of 23.34.

‘Difference-in-difference analysis’ was then used to evaluate the trend of COVID-19 infections before and after a mask mandate was introduced. Test counties were paired with control counties that had similar populations and were from the same state. The researchers then analyzed the infection rates from 10 days before the mask mandate was introduced in the test county until 30 days after, and compared the rates in the test counties with those of the controls.

Ten days before the mask mandate was introduced, the test counties had a similar infection rate to controls (16.05 versus 14.01 average cases per day). At 30 days after the mandate’s introduction, however, COVID-19 cases had dropped by 4.22 cases per day in test counties, a 16.9% reduction according to the difference-in-difference analysis.

These findings are particularly useful given that, in those US states without statewide mask mandates, measures to control the spread of COVID-19 are decided at county level. The study’s results can be used by local government officials to decide whether to impose mask mandates and by doctors to better inform their patients on the effectiveness of mask wearing.

“With the Delta variant and, more recently, the Omicron variant of COVID-19, vaccine effectiveness is slowly reducing,” says lead author Hadie Islam. “This study provides significant evidence for public health and medical professionals to encourage the wearing of masks to prevent the spread of COVID-19.”

The paper can be found here:

De Gruyter
 Mauricio Quiñones
 Tel: +49 30 260 05 164

De Gruyter publishes first-class scholarship and has done so for more than 270 years. An international, independent publisher headquartered in Berlin -- and with further offices in Boston, Beijing, Basel, Vienna, Warsaw and Munich -- it publishes over 1,300 new book titles each year and more than 900 journals in the humanities, social sciences, medicine, mathematics, engineering, computer sciences, natural sciences, and law. The publishing house also offers a wide range of digital media, including open access journals and books. The group includes the imprints De Gruyter Akademie Forschung, Birkhäuser, De Gruyter Mouton, De Gruyter Oldenbourg, De Gruyter Saur, Düsseldorf University Press, Deutscher Kunstverlag (DKV) and Jovis Verlag, as well as the publishing services provider Sciendo. For more information, visit:

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