The Centers for Disease Control and Prevention on Tuesday cited new science on the transmissibility of the delta variant in revising its mask guidance to now recommend that everyone in areas with substantial or high levels of transmission -- vaccinated or not -- wear a mask in public, indoor settings.
The agency also called for universal masking in schools.
The new science, gathered from several states and other countries, shows that "in rare occasions, some vaccinated people infected with a delta variant after vaccination may be contagious and spread the virus to others," CDC Director Rochelle Walensky told reporters during a briefing on Tuesday afternoon.
"This new science is worrisome, and unfortunately warrants an update to our recommendation," Walensky said.
She said the new data emerged in the last week and showed that vaccinated people who are infected with the delta variant could carry the same viral load as unvaccinated, infected people. As a result, the CDC is asking that even the vaccinated wear masks in public, indoor settings "to help prevent the spread of the delta variants and protect others."
Walensky emphasized that the vast majority of transmission is still happening among unvaccinated people, and the best way forward is to increase vaccinations everywhere because the vaccines were still considered to be highly effective. The risk of a symptomatic infection is reduced sevenfold for fully vaccinated people, Walensky said, and the risk of hospitalization is reduced twentyfold.
"Getting vaccinated continues to prevent severe illness, hospitalization and death, even with delta. It also helps reduce the spread of the virus in our community. Vaccinated individuals continue to represent a very small amount of transmission occurring around the country," Walensky said.
The public health agency also recommended schools embrace universal masking, departing from guidance released earlier this month that suggested vaccinated students and staff were safe to go without a mask.
"CDC recommends localities encourage universal indoor masking for all teachers, staff, students, and visitors to schools, regardless of vaccination status," the CDC wrote in a summary of the new guidance. "Children should return to full-time in-person learning in the fall with proper prevention strategies are in place."
The revised guidance raised questions about the CDC's previous decision to no longer recommend masking, indoors or outdoors, for fully vaccinated people, and whether that was an overly confident move that relied too heavily on an honor system for unvaccinated Americans to continue to follow the rules.
But it also raises questions about how likely it is that fully vaccinated people can transmit the virus when infected, and whether that has changed with the onset of the delta variant -- a question that the CDC is still studying but has said with increasing confidence is much less likely to occur among vaccinated people as opposed to unvaccinated people.
Emerging science on that question was a key reason the CDC changed the mask guidance two months ago, in May, when it announced that all vaccinated Americans were safe to go without a mask indoors or in a crowd. Its guidance for schools followed that principle.
CDC recommendations noted that individuals and schools could still opt to wear a mask even if fully vaccinated, but said the risk of illness and transmission was low.
At the time, Walensky pointed to a "coalescence of more science that has emerged just in the last week" in three areas.
But evolving factors have forced the CDC to take another look; chief among them, the delta variant, and as a close second, some Americans' unwillingness to get vaccinated. And while the CDC didn't predict those barriers, Walensky has consistently reminded Americans that guidance will have to change as the pandemic does.
"This past year has shown us that this virus can be unpredictable. So, if things get worse, there is always a chance we may need to make a change to these recommendations," she said on May 13.
On Tuesday, she said she was dismayed to come forward with a new recommendation that certain swaths of the country re-implement mask mandates indoors, but that experts agreed that the new data "required action."
"This weighs heavily on me. I know at 18 months through this pandemic, not only are people tired, they're frustrated," Walensky said. "And I know, in the context of all that, it is not a welcomed piece of news that masking is going to be a part of peoples lives who have already been vaccinated."
But she also pointed to low vaccination rates as the reason the delta variant was forcing the change in guidelines in the first place, urging people to step up and get a shot.
"This moment, and most importantly, the associated illness, suffering and death could have been avoided with higher vaccination coverage in this country," Walensky said. She warned, too, that if vaccinations don't increase, the "big concern" is that the virus will continue to spread, mutate and eventually "could potentially evade our vaccines."
As of Tuesday, about 69% of adult Americans had at least one shot, while 60% were fully vaccinated. The country missed President Joe Biden's goal to have at least 70% of adults with one shot by July 4.
And the story varies widely across the country, particularly in pockets in the southeast and midwest, where vaccination rates dip below the national average.
On Tuesday, the CDC told reporters that those factors didn't change the benefits of getting vaccinated: the risk of severe illness from COVID still remains low for Americans who are fully vaccinated and the vast majority of people hospitalized with COVID-19 are unvaccinated.
But the delta variant, which has taken root in the U.S. over the last month and now represents 83% of all infections, is different than past mutations of the virus, the CDC said.
Doctors and researchers who have been tracking the pandemic and working on the frontlines were largely supportive of the CDC's decision on Tuesday.
"We are seeing the delta variant cause a spike in hospitalizations in the U.S., just like it did in the U.K. and India earlier this summer. A return to indoor masking is a simple way to slow the spread," said Caitlin M. Rivers, an epidemiologist and senior scholar at The Johns Hopkins Hospital.
And on the question of transmission among vaccinated people, the question should be framed up against the much greater risk, which is transmission among people who have not gotten a vaccine and have ceased following any safety guidelines, said Dr. William Schaffner, a professor of preventive medicine and infectious diseases at Vanderbilt University in Nashville.
"The issue is the unvaccinated. That's where the transmission is like a four-lane highway with all that traffic. There's some spillover to the vaccinated, but that's like a bunch of side streets," Schaffner said.
"Yes, there is transmission among the vaccinated and from the vaccinated, but it's very low in comparison to the amount of transmission that is occurring among unvaccinated people."
But the unknowns still increases concerns for parents of young children who aren't eligible for vaccines yet.
"Parents of unvaccinated children should have their kids wear a mask, avoid crowds and continue to take the precautions we've been relying on throughout the pandemic to reduce risk," Rivers said.
Rivers said she had returned to wearing her mask indoors, regardless of whether she was in an area with high transmission -- and Schaffner said he'd never stopped.
"I am back in a mask when indoors in public, even though I'm fully vaccinated," Rivers said.
Schaffner compared it to wearing a belt and suspenders to hold up pants -- double protection.
"I'm an infectious disease doc, I really respect this virus, and I will take every layer of protection that I can get," he said.
He encouraged the general public, no matter where they live, to take up mask-wearing again when indoors.