New information from a big study helped prompt the updated guidance, the agency says. Here's what we know about the data.

"This report provided the most compelling information to date that COVID-19 vaccines were performing as expected in the real world," Walensky said in a statement Friday.
"COVID-19 vaccines are effective at preventing COVID-19 disease, especially severe illness and death," CDC says on i ts new web page describing guidance for the fully vaccinated.
"COVID-19 vaccines reduce the risk of people spreading COVID-19."
After weeks of telling people that even fully vaccinated people might carry virus in their noses, mouths or throats and breathe or spit it out onto others, the CDC says the evidence shows this is unlikely.
At least three major studies have shown that fully vaccinated people are not likely to test positive for coronavirus, which indicates they are not carrying it in their bodies, whether they have symptoms or not.
Last March 29, a network of researchers released a study via the CDC that involved nearly 4,000 health care workers who tested themselves weekly.
"This study, added to the many studies that preceded it, was pivotal to CDC changing its recommendations for those who are fully vaccinated against COVID-19."
According to CDC data, nearly 47% of the US population has received at least one dose of Covid-19 vaccine and about 36% of the US population is fully vaccinated.
In theory, if only fully vaccinated people went mask-free, there should be little risk of the virus spreading.
Some critics have complained that the CDC has made no provision for people who have not been vaccinated but who will not wear masks.
And President Joe Biden confirmed that Americans will be on the honor system for making sure they are vaccinated before discarding face masks.
Back in April, when the CDC said fully vaccinated people should still wear masks inside when around other people, the agency did note that encouraging people to get vaccinated might be a legitimate factor in changing guidance.