WASHINGTON, United States -- As COVID-19 infections continued to rise, the US Centers for Disease Control and Prevention (CDC) has updated its position on masking, urging vaccinated Americans to resume wearing masks in schools and in public indoor spaces in COVID-19 hot spots across the country.
The new guidance marks a sharp turnabout from the agency's position since May that vaccinated people do not need to wear masks in most indoor spaces.
"This is not a decision we at CDC have made lightly. This weighs heavily on me," said CDC Director Rochelle Walensky.
"The Delta variant is showing every day its willingness to outsmart us," she said. "In rare occasions, some vaccinated people infected with a Delta variant after vaccination may be contagious and spread the virus to others."
Data from several states and other countries show that the Delta variant behaves differently from previous versions of the coronavirus.
"This new science is worrisome and unfortunately warrants an update to our recommendation," Walensky said.
The Delta variant has quickly grown from less than 1 percent of cases in May to more than 80 percent now, said the CDC on its website.
Delta spreads about twice as easily from one person to another than previous strains of the virus, according to the agency.
Most transmission happening around the country is among unvaccinated people and in areas with low vaccination rates.
Health officials and experts have been urging more people to get vaccinated to stay ahead of changes in the virus.
The CDC's new mask guidelines met a mix of support and dismay from around the country.
Some states health departments moved immediately to adopt the guidelines, such as Illinois, where health officials recommended masks for everyone in public indoor spaces in areas with high rates of transmission, regardless of vaccination status.
While in some other states, conservative politicians and their supporters have cast public health measures as an attack on freedom.