The U.S. Centers for Disease Control and Prevention (CDC) said on Wednesday that 66.6 percent of U.S. counties had transmission rates of COVID-19 high enough to warrant indoor masking and should immediately resume the policy.

The transmission rate was up from 63.4 percent as of Tuesday. In total, 49.9 percent of U.S. counties have high COVID-19 community transmission rates and 16.7 percent have substantial rates, the CDC said. The CDC reversed its mask guidance policy for vaccinated Americans on Tuesday.

Substantial transmission means at least 50 new cases per 100,000 people in the last seven days, while high transmission is more than 100 cases per 100,000 people over the last week.

In a toughening of guidance issued earlier this month, the CDC also recommended all students, teachers and staff at schools for kindergarten through 12th grade wear masks regardless of whether they were vaccinated.

U.S. coronavirus cases have been rising due to the highly contagious Delta variant, which was first detected in India but has quickly spread and now accounts for more than 80 percent of U.S. coronavirus cases.

However, the new CDC recommendations are not binding and many Americans, especially in Republican-leaning states, may choose not to follow them.