The US authorities have recommended vaccinated people to wear masks indoors again in high-risk corona areas within the USA.
The CDC health authority justified the recommendation on Tuesday with the increasing number of corona infections as a result of the highly contagious Delta variant.
"For areas with significant and high transmission rates, the CDC recommends fully vaccinated people to wear a mask in public indoor spaces," said agency director Rochelle Walensky.
The effectiveness of the vaccines is also high in the Delta variant, emphasized Walensky.
However, recent data indicated that "on rare occasions, some vaccinated people can be contagious and transmit the virus to others".
Walensky described the new scientific findings as "worrying".
According to data from the CDC, large parts of the southern United States have a significant or high transmission rate.
In the northeast of the country, where the vaccination rate is higher, it is lower.
The delta variant is now responsible for around 90 percent of cases of infection.
49 percent of the US population is fully vaccinated so far.