FIRST ON FOX: A group of House Republicans is pushing for more transparency for the Centers for Disease Control and Prevention (CDC) on the data behind its most recent mask guidance for vaccinated people and what metrics it will need to see to rescind the controversial recommendation.

Rep. Mike Gallagher, R-Wis., is leading the letter, which was first obtained by Fox News, with 46 other House Republicans. Among them are GOP Conference Chair Elise Stefanik, R-N.Y., and Republican Study Committee Chairman Rep. Jim Banks, R-Ind. The letter comes amid controversy over renewed state and local universal mask mandates based on the new CDC guidance even though COVID-19 vaccines are highly effective at preventing severe illness, hospitalization and death. They also significantly reduce the transmission of the virus.

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“Unfortunately, the CDC has not been transparent with the data underlying this decision, sowing confusion and distrust,” the letter reads. “So, we write to ask that you publicize this data and explain how it justifies a blanket and open-ended policy.”

“If your agency cannot even provide reliable predictions about current transmission, we question your ability to estimate the extent to which breakthrough infections are expected to drive the overall number of cases among the unvaccinated,” the GOP group adds. “We worry this shifting mask guidance and lack of transparency signals the CDC does not have a plan to show the American people what benchmarks need to be achieved to bring their lives back to normal.”

The letter asks the CDC to provide data on breakthrough infections; how those infections will drive overall coronavirus case numbers; the specific data that led to the CDC’s masking decision; and what metrics the agency would need to see in order to remove its masking guidance for the vaccinated.

A CDC official familiar with the process told Fox News on Thursday that the agency is going to be releasing the data behind its decision on Friday.

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CDC Director Rochelle Walensky has said that the unpublished data the agency has seen – which the Associated Press reported comes from 100 samples from several states and another country – shows that vaccinated people infected with the delta variant have much higher viral loads than other breakthrough infections.

This could be an indicator of potential transmissibility in people infected with the variant. But it is unclear whether the CDC has data specifically proving the transmissibility of the delta variant, rather than just that circumstantial evidence.

Rep. Mike Gallagher, R-Wis., conducts a news conference after a meeting off the House Republican Conference in the Capitol Visitor Center on Tuesday, June 4, 2019. Gallagher led a group of House Republicans Friday in pushing for more transparency on the CDC’s goals and data behind its new masking guidance. (Photo By Tom Williams/CQ Roll Call)

Further, the current CDC recommendations do provide a metric for when vaccinated people should be wearing masks and when they can leave them at home. The agency says that people in counties with “substantial” or “high” rates of COVID-19 transmission should wear masks in indoor public settings.

This means that in places where there are 50 or more cases per 100,000 people over 7 days, the CDC says vaccinated people should wear a mask. In places with lower case rates, the CDC recommendations do not apply.

The CDC did not immediately respond to a request for comment from Fox News on what metrics it would need to see in order to rescind the new mask guidance, including whether leaving in place the current guidance with its built-in metrics indefinitely is on the table.

The White House Thursday declined to provide any timetable or metrics on when the mask guidance will be rescinded.

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“There’s not a goal or… anything like that we’re putting forth,” White House Deputy Press Secretary Karine Jean-Pierre said at a press briefing. “What we want to make sure is that we get our message out there and make sure people are getting vaccinated. That is what the science tells us, that’s how we’re going to curb this pandemic is making sure that people get vaccinated.”

Asked whether Americans should trust the president that the U.S. is not going back to lockdowns or school closures, Jean-Pierre responded: “Well, because we listen to the scientists; we listen to the expert.”

“This is a public health situation. This is not about politics at all. This is about saving lives,” she said.

Fox News’ David Lewkowict contributed to this report.