Tennessee counties that have not required wearing masks in public are on average seeing COVID-19 death rates double or more compared with those that instituted mandates, according to a report released Tuesday.

The Vanderbilt University School of Medicine study focused on three groups of counties: 11 early adopters with mask mandates as of July 10; 17 late adopters with mandates implemented after July 10; and 67 those that never adopted a requirement.

Researchers found the early and late adopting groups saw death rates that had been increasing start to drop within a few weeks of implementing requirements, while the group with no requirements continued to see death rate increases.

The early adopters on average had a rate of about 1 death a week per 100,000 people as of the first week in October; late adopters’ death rate was about 2; and the counties without mask mandates had a rate of 4, according to the report.

The analysis comes as new case counts rise in Tennessee, where Republican Gov. Bill Lee has opposed a statewide mask mandate, stressing personal responsibility. He has instead allowed counties to decide whether to require masks.

The study groups the data by the date of death, not the date it was reported, which can lag the actual date of death by several weeks. The deaths likely stem from infections likely in mid- to late September, predating the recent rise in cases, researchers noted. Other policies and behaviors were also factors, they said.

“This analysis shows that strategies, including but not limited to masking while in contact with others, can have real impact on people’s lives,” John Graves, director of the Vanderbilt Center for Health Economic Modeling, said in a news release. “Mask mandates are associated with greater mask wearing and other behaviors like limiting close contacts with others, and the combined impact is clear and substantial.”

The study follows related research by Vanderbilt late last month, which found sharper increases of patients in Tennessee hospitals in recent weeks due to COVID-19 in areas without mask mandates.

The report says 63% of Tennesseans are currently required to wear masks, 31% never were under a mask requirement and 6% only faced one in the summer months.

The 7-day rolling average of daily deaths in Tennessee has held steady, from 34.43 on Oct. 26 to 33 on Monday. Meanwhile, the 7-day rolling average of daily new cases for the state has risen from about 2,544 on Oct. 26 to 3,312 on Monday.

For most people, the coronavirus causes mild or moderate symptoms that clear up within weeks. But for others, especially older adults and people with existing health problems, the virus can cause severe symptoms and be fatal. The vast majority of people recover.