Federal health officials still believe fully vaccinated individuals represent a very small amount of transmission. Still, some vaccinated people could be carrying higher levels of the virus than previously understood and potentially transmit it to others.

"This pandemic continues to pose a serious threat to the health of all Americans," CDC Director Rochelle Walensky told reporters on a call. "Today, we have new science related to the delta variant that requires us to update the guidance regarding what you can do when you are fully vaccinated."

The updated guidance comes ahead of the fall season, when the highly contagious delta variant is expected to cause another surge in new coronavirus cases and many large employers plan to bring workers back to the office. In mid-May, the CDC said fully vaccinated people didn't need to wear masks in most settings, whether indoors or outdoors.