U.S. communities that mandated the use of face masks in public saw an ongoing decline in the spread of coronavirus, according to a study reported by CNN on Monday.

After five days of mask mandates, the daily coronavirus growth rate slowed by just under 1 percent, while after 21 days, the growth rate slowed by about 2 percent, researchers from the Department of Health Management and Policy at the University of Iowa College of Public Health reported in the journal Health Affairs.

Between April 8 and May 15, governors of 15 U.S. states and the mayor of Washington, D.C. signed orders to place mask mandates in public. The researchers studied changes in the daily county-level COVID-19 growth rates between March 31 and May 22.

The researchers also projected the number of averted COVID-19 cases with the mandates for face mask use in public by comparing actual cumulative daily cases with daily cases predicted by a model if none of the states had enacted the public face cover mandate at the time they did.

The model estimates suggested that 230,000-450,000 cases may have been averted by May 22 due to these mandates.