The CDC researchers looked at U.S. counties placed under state-issued mask mandates and at counties that allowed restaurant dining — both indoors and at tables outside. The study looked at data from March through December of last year.

The scientists found that mask mandates were associated with reduced coronavirus transmission, and that improvements in new cases and deaths increased as time went on.

The reductions in growth rates varied from half a percentage point to nearly 2 percentage points. That may sound small, but the large number of people involved means the impact grows with time, experts said.

“Each day that growth rate is going down, the cumulative effect — in terms of cases and deaths — adds up to be quite substantial,” said Gery Guy Jr., a CDC scientist who was the study’s lead author.