More than a day after issuing new guidance that vaccinated people should wear masks indoors – recommendations that are likely to affect millions of Americans in the form of private and public mask mandates – the Centers for Disease Control and Prevention has yet to release the data behind its decision. 

The lack of clarity on what exactly the agency is basing its decision on comes as states and cities across the country are working to vaccinate more of their citizens. And it's making some local officials reluctant to follow the CDC's advice on nearly universal masking.


"While the CDC issued their guidance yesterday at about 3 p.m., they have not yet released their scientific reports on the data that underlies their recommendation," New York City Health and Hospitals President and CEO Mitchell Katz said at a Wednesday press conference with Mayor Bill de Blasio. 

Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention, adjusts her face mask during a Senate Health, Education, Labor and Pensions Committee hearing on the federal coronavirus response on Capitol Hill in Washington, in this Thursday, March 18, 2021, file photo.  (AP Photo/Susan Walsh, Pool)

"I think we owe it to New Yorkers to very carefully, as you say, review that information and understand its implications," Katz added. "Our focus has to be on getting people vaccinated." 


"We're assessing the information. What really is important is to assess the research behind it. Which is what our team is doing," de Blasio said when asked why the city is delaying any action to mandate mask-wearing for vaccinated folks. "We've got to make sure we understand the ramifications and what makes sense to do."

CDC Director Rochelle Walensky explained that the reason for the CDC's reversal on indoor masking for vaccinated people is because "in rare occasions, some vaccinated people infected with the delta variant after vaccination may be contagious and pass the virus to others."

They "may be contagious," Walensky said, because "the amount of virus" in vaccinated people infected by the delta variant "is pretty similar to the amount of virus in unvaccinated people." 

Mayor of New York City Bill de Blasio attends the opening of a vaccination center for Broadway workers in Times Square on April 12, 2021 in New York City. De Blasio and his top health officials Wednesday were reluctant to re-implement a mask mandate without data from the CDC to support its new guidelines.  (Noam Galai/Getty Images)

According to the Associated Press, the data to support this claim emerged over the last couple of days from over 100 samples from several states and one other country. The CDC has not released the data yet, and it is not public. 

Rep. Peter Meijer, R-Mich., Wednesday criticized the CDC for its lack of transparency, and the Capitol attending physician for issuing a House mask mandate without seeing the CDC's underlying data.

"I think a lot of the frustration is coming with just the blanket absorption of CDC guidance that shifted based upon what they're citing as two unpublished studies that by virtue of being unpublished no one else can look at," he said. Meijer als noted the CDC is citing "a third study from India that was based off of delta transmissibility among individuals who had a non-U.S. vaccine."


"The vast majority of transmission is occurring through unvaccinated individuals," Walensky said Tuesday. But, she added, "we thought it was important for [vaccinated] people to understand that they could pass the disease onto someone else." 

CDC guidance published Tuesday cited "unpublished data" reviewed by its COVID-19 Response Team.

I think a lot of the frustration is coming with just the blanket absorption of CDC guidance that shifted based upon what they're citing as two unpublished studies that by virtue of being unpublished no one else can look at.

— Rep. Peter Meijer, R-Mich.

"We anticipate the data being released soon," CDC spokesperson Kristen Nordlund told Fox News Thursday. A CDC official familiar with the process said later Thursday that the first publication of the CDC's data will happen Friday.

But the CDC did not reply to a separate request for comment from Fox News Wednesday about the specific data behind its mask guidance or whether it has evidence of actual transmission from vaccinated people. 

The CDC also did not address whether it is worried about mass non-compliance with its new guidelines as even some high-profile Democrats like de Blasio, Pennsylvania Gov. Tom Wolf, and Michigan Gov. Gretchen Whitmer hesitate to bring back masking rules. 

The CDC cannot itself mandate mask-wearing — state and local governments do. But many state and local governments closely follow the CDC's guidance on the coronavirus and some immediately adopted mask mandates after the CDC announcement Tuesday, including Nevada and Kansas City. 


"I have stuck with CDC guidance throughout the pandemic and today is no different," Kansas City, Missouri Quinton Lucas said. "I will return Kansas City to a mask mandate indoors based upon national and regional health guidance and discussion with other Kansas City leaders. I will provide further details in the morning."

The new CDC mask guidance comes despite the fact that COVID-19 vaccines are extremely effective at preventing severe illness, hospitalization and death. The vaccines also massively reduce the risk of virus transmission.

Fox News' Kayla Rivas, David Lewkowict and the Associated Press contributed to this report.