The Centers for Disease Control and Prevention issued new guidance on Tuesday recommending indoor mask use in areas with high transmission rates after new data suggested fully vaccinated individuals are not just contracting COVID-19 but could potentially infect others.
CDC Director Rochelle Walensky said recent studies had shown that those vaccinated individuals who do become infected with COVID have just as much viral load as the unvaccinated, making it possible for them to spread the virus to others. Based on that finding, Walensky said the CDC is also recommending that all school children wear masks in the fall.
"We're seeing now that it's actually possible if you're a rare breakthrough infection that you can transmit further, which is the reason for the change," Walensky said.
Administration officials still contend that the fully vaccinated represent a very small percentage of transmission, which is primarily occurring among unvaccinated people. Walensky said the decision to call a return to mask use wasn't taken lightly, and that she hopes it will be temporary — until the ranks of the vaccinated increase, and the amount of virus circulating in the community decreases.
"It is not a welcomed piece of news that masking is going to be a part of people's lives who have already been vaccinated," Walensky said. "This new data weighs heavily on me, this new guidance weighed heavily on me and I just wanted to convey that this was not a decision that was taken lightly."
The guidance comes after an internal debate among health officials: Whether to respond to these findings by simply informing the public about them or by recommending additional restrictions, including a return to uniform indoor mask use for both vaccinated and unvaccinated individuals.
Administration officials had been struggling in recent days with how to respond to the rising number of COVID-19 infections and hospitalizations among those who are fully vaccinated amid pressure from public health experts to put mask recommendations back in place. Much of Biden's domestic agenda rides on moving the country past the pandemic and shifting the focus to other priorities, such as infrastructure and voting rights.
Biden said in a statement following the announcement that he would follow the guidance when he travels, and encouraged others to do the same. He also called on more Americans to get vaccinated and said he would detail further plans on Thursday to increase the vaccination rate.
"Today’s announcement by the CDC — that new research and concerns about the delta variant leads CDC to recommend a return to masking in parts of the country — is another step on our journey to defeating this virus. I hope all Americans who live in the areas covered by the CDC guidance will follow it. I certainly will when I travel to these areas," Biden said.
Within the administration, there had been concern that a focus on mask use could take away a key incentive for people to get vaccinated, which they believe was a factor for many people in choosing to get the shot. Some argued the best way to keep the virus under control was to double down on efforts to get as many vaccinated as possible.
“I understand how challenging this is in terms of the vaccine hesitancy and wanting people to get the vaccine,” said Dr. Leana Wen, a former Baltimore health commissioner and a professor and physician at Georgetown University. “But the Biden administration made a serious error in the first place with their CDC guidance on masks and it is really hard to put the genie back in the bottle.”
White House press secretary Jen Psaki said Monday the president will follow all CDC guidance around wearing masks when traveling to places where they are recommended.
While the COVID-19 vaccines have led to a steep decline in new infections, deaths and hospitalizations, multiple studies have found they are less effective against the new delta variant that now accounts for the vast majority of infections in the United States.
With the virus still widely circulating in many places where there are low vaccination rates, that has put the vaccinated at increased risk the virus will break through the layer of protection. In some states, particularly in the South, just over a third of the population is fully vaccinated.
Given how infectious the variant is, some health experts are now also questioning the effectiveness of standard cloth masks and advocating for more effective masks like the KN95 to be recommended for indoor use for all, particularly among the elderly and the immunocompromised. One study found those infected with the delta variant carried 1,000 times the virus as with earlier strains.
The worrisome data comes just weeks after the Centers for Disease Control and Prevention said vaccinated individuals can go about their business without masks. Administration officials had been concerned a return to widespread mask mandates could actually stunt the broader vaccination drive by taking away an incentive for getting the shots.
Despite a push over the last two month by the Biden administration to increase the number of people getting vaccinated, the number of daily shots has plateaued at around 500,000 per day, and Biden has yet to reach his July 4 goal of getting 70 percent of adults at least partially vaccinated.
Just how widespread infections among the vaccinated are in the U.S. is unknown. The CDC said in May it would stop monitoring the number of infections in vaccinated people aside from cases where a fully vaccinated person was hospitalized or died. Walensky said on Monday the agency has been tracking specific groups for breakthrough infections and would be reporting that data soon.
The limited data so far has left doctors saying they feel they are flying blind in trying to assess the risk the new variant poses to their patients and relying on data out of other countries like Israel, where researchers released data last week showing the Pfizer-BioNTech vaccine was just 39 percent effective against preventing infection from the delta variant and 91 percent effective at preventing severe disease.
“It is a very short-sighted policy to not be collecting that data because they are worried that it will be misinterpreted by people who think vaccines don’t work,” said Christopher Murray, director of the Institute for Health Metrics and Evaluation at the University of Washington. “It is essentially making us a little blind to what is actually going on.”
But despite the lack of data from the CDC, anecdotal evidence is widespread, with high-profile cases of groups of fully vaccinated individuals getting infected — like a group of Democrats from the Texas Legislature, a White House official, a group of New York Yankees, and an Olympic athlete.
Michael Osterholm, director of the Center for Infectious Disease Research at the University of Minnesota who advised the Biden transition, said he is particularly concerned about the increasing number of instances he is hearing about where groups of vaccinated individuals are becoming infected. The CDC has said the data indicates the vaccinated can’t spread the virus to other vaccinated people, but Osterholm said the clusters of cases among the vaccinated suggest otherwise.
In one cluster of more than 200 cases in Provincetown, Massachusetts, 70 percent of the COVID-19 positive cases since July 1 have been among vaccinated individuals, Town Manager Alex Morse said in an MSNBC interview.
A small percentage of those who are fully vaccinated also continue to be hospitalized and die from the virus, particularly in areas seeing a surge in cases from the delta variant.
At the Texas Medical Center in Houston, 10 percent of more than 250 hospitalized COVID-19 patients were vaccinated, a hospital spokesperson said. In Las Vegas, 11 percent of Covid hospitalizations in July had been fully vaccinated people, according to the Southern Nevada Health District. A spokesperson for University of Kansas Health System said July 15 that 16 percent of inpatients were fully vaccinated people.
Hospital officials added, though, that those who are vaccinated are less likely to be in the ICU or die, and nearly all who were admitted after vaccinations had serious underlying conditions.
Nationwide, the CDC has tracked more than 4,000 cases of fully vaccinated people being hospitalized with symptoms of COVID-19 as of July 19, up 9 percent from the week earlier. The agency said a total of 849 vaccinated people have died, including 58 the week of July 12, according to data reported to the CDC by local health departments.
This story originally appeared on NBCNews.com.