On Tuesday (July 27), the Centers for Disease Control and Prevention (CDC) updated their mask guidance, saying fully vaccinated people should resume wearing masks in public indoor spaces if there is substantial coronavirus transmission in their area.
"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," Dr. Rochelle Walensky, the CDC director said at a media briefing on Tuesday (July 27). The delta variant behaves "uniquely differently" from past variants, she said.
Data from the U.S. and other countries indicates that some vaccinated people infected with the delta variant may be contagious and spread the virus to others, she said.
Related: Delta variant: Your questions answered
Unpublished data suggests that vaccinated people infected with the delta variant carry about the same viral load as those who are unvaccinated, which was not the case with earlier coronavirus variants. In other words, people with breakthrough infections have the potential to transmit SARS-CoV-2 to others "with the same capacity" as those who are unvaccinated, Walensky said in response to a question at the briefing. Other research, which has not yet been peer-reviewed, suggested that people infected with the delta variant may have 1000-fold higher viral load than did people infected during the original outbreak in Wuhan, Live Science previously reported.
Still, breakthrough infections are uncommon and most coronavirus transmission is occurring through unvaccinated individuals, Walensky said.
"I think the most important thing to understand is the vaccines continue to do an exceptional job in preventing an individual who is vaccinated from severe illness, hospitalization and death — and even against mild illness," Walensky said. But "we felt it important for people to understand that they have the potential to transmit the virus to others" if they do get a breakthrough infection with the delta variant.
If exposed to the delta variant, COVID-19 vaccines are thought to reduce the risk of a symptomatic breakthrough infection by sevenfold and hospitalization and death by 20-fold, she said.
"CDC has recommended for months, unvaccinated individuals should get vaccinated and continue masking until they are fully vaccinated," Walensky said. In May, the CDC updated its mask guidance to say that fully vaccinated people can go to most indoor and outdoor areas without masks, Live Science previously reported.
But the newly updated guidelines hold that in areas with "substantial" or "high" coronavirus transmission, fully vaccinated people should resume wearing masks in public indoor settings "to help prevent the spread of the delta variant and protect others," Walensky said.
Substantial transmission means that a county is reporting between 50 to 100 cases per 100,000 over a week and high transmission means that a county is reporting more than 100 cases per 100,000 over a week. In some counties, cases have reached more than 300 per 100,000 over a week, which "we're concerned about," Walensky said. (You can track your county transmission rate on the CDC website.)
The new recommendations include schools; everyone in K-12 schools should wear a mask indoors, including teachers, staff, students and visitors regardless of vaccination status, she said.
"With the delta variant, vaccinating more Americans now is more urgent than ever," Walensky said. "The highest spread of cases and severe outcomes is happening in places with low vaccination rates and among unvaccinated people."
Originally published on Live Science.