A new study is showing how mask mandates affect COVID-19 case numbers during the pandemic.

Doctors from the University of Pennsylvania and Children’s Hospital of Pennsylvania looked at case numbers in counties weeks after mask mandates were implemented.

The study spanned between March and October of 2020, during the time cases were peaking across the country.

Researchers found that, on average, daily cases declined by 23% four weeks after mask mandates were implemented, in comparison to counties that did not have mask mandates.

Case numbers improved as time went on.

Daily cases decreased by 33% six weeks after mask mandates were implemented, compared to unmasked counties.

The study goes on to show that mask mandates were more effective in urban counties.

The study comes as the CDC prepares to issue new guidance on masks.