Treasury Secretary Janet Yellen said Friday the U.S. could return to lockdowns in areas of the country with low COVID-19 vaccination rates.

“We have solid rates of vaccination in many parts of the country. But certainly it’s something that could happen in areas where vaccination rates are low,” she told CNBC.

COVID-19 infections have risen in the U.S. recently after months of decline.

As of July, confirmed cases reached an average of 23,600 infections a day, according to Fox Business.

Yellen said that to prevent further lockdowns, it “is critically important that we maintain progress on vaccinating more Americans.”