July 27 (UPI) -- The Centers for Disease Control and Prevention on Tuesday recommended that vaccinated Americans again start wearing masks indoors, citing new scientific data on how the Delta variant of COVID-19 spreads.
CDC Director Dr. Rochelle Walensky told reporters the Delta variant -- which is now the dominant COVID-19 variant in the United States -- "behaves uniquely differently" compared to earlier strains.
"The Delta variant is showing every day its willingness to outsmart us," she said.
In "rare" cases in which a fully vaccinated person is infected with the variant, they may be able to spread the virus, she added. To prevent that spread, the CDC updated its guidelines to encourage fully vaccinated people to wear a mask in indoor public spaces in areas of high transmission of COVID-19.
Walensky also said anyone inside a K-12 school -- students, teachers, administrators and visitors -- should wear a mask regardless of vaccination status.
The move is a step back for the CDC after the agency said in May that vaccinated Americans no longer needed to wear masks indoors.
Walensky said the masking announcement was "not a decision that we ... made lightly."
"This weighs heavily on me 18 months into this pandemic," she said, adding that people are tired, frustrated and battling mental health challenges.
"I know in the context of all that, it is not a welcome piece of news that masking is going to be a part of life for people who are already fully vaccinated."
The change has been driven by the spread of the Delta coronavirus variant, which is a more contagious version of COVID-19.
The coronavirus has made a return to many states -- such as those in the South that have low vaccination rates and Western states such as Arizona and Wyoming.
According to the latest data from the CDC, almost half of all counties in the United States are now listed as a "high" risk area for coronavirus transmission. That share is almost 20% higher than it was seven days ago.
Another 17% of U.S. counties are listed as having a substantial transmission risk.
State health officials and the CDC have said that 95% of COVID-19 patients who are being hospitalized were not vaccinated. Despite the greater spread of the Delta variant, even among the vaccinated, Walensky said vaccines reduce symptomatic cases of the Delta variant by seven-fold and hospitalizations by 20-fold.
Hospitals in Alabama, Louisiana and Missouri have reported crowded conditions in intensive-care units due to the rise in cases. Patients have ranged in age from 30 to 60.