Updated

Capitol Attending Physician Brian Monahan said late Tuesday that the House of Representatives is reinstating its mask mandate – and therefore the threat of fines to members who don't comply – following updated guidance from the Centers from Disease Control and Prevention (CDC) on the delta variant of the coronavirus.

The White House also appears to be going back to mandated masks. A White House press representative was seen Tuesday swapping a sign saying people are required to wear masks if unvaccinated with another saying masks are required regardless of vaccination status. 

The CDC said Tuesday that it is recommending people who are vaccinated and in areas with high COVID-19 spread wear masks indoors. The announcement was foreshadowed for days by top officials and demanded for weeks by some activists. The new recommendation is likely to prompt a raft of new mask mandates around the country and has already done so in the U.S. Capitol. 

"For the Congress, representing a collection of individuals traveling weekly from various risk areas (both high and low rates of disease transmission), all individuals should wear a well-fitted, medical-grade filtration mask (for example an ear loop surgical mask or a KN95 mask) when they are in an interior space," Monahan said in a letter sent to congressional staffers. 

This, the attending physician added, includes the House chamber, committee meetings and all House office buildings. The only explicit exceptions Monahan mentioned were for those who are actively speaking, seeking recognition from a chair at a committee hearing or those who are alone in a room. 

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"To be clear, for meetings in an enclosed US House of Representatives controlled space, masks are REQUIRED," Monahan said. "Failure to wear a mask in the Hall of the House is subject to fines imposed for violation as contained in the previous House rule."

Monahan explained the reasoning behind the decision in his letter, tracking closely with reasoning used by CDC Director Rochelle Walensky in his announcement Tuesday that the CDC would again recommend masks for unvaccinated people. 

Speaker of the House Nancy Pelosi, D-Calif., wears a face mask as she hosts a visit by King Abdullah II of Jordan at the Capitol in Washington, Thursday, July 22, 2021. 

Speaker of the House Nancy Pelosi, D-Calif., wears a face mask as she hosts a visit by King Abdullah II of Jordan at the Capitol in Washington, Thursday, July 22, 2021.  (AP Photo/J. Scott Applewhite)

"In the past two weeks, USA coronavirus cases have significantly increased, now approaching 100,000 cases per day. The delta variant virus has been detected in Washington, DC and in the Capitol buildings," Monahan said. "It represents a dire health risk to unvaccinated individuals and is not without some risk to the vaccinated individuals or their unvaccinated household contacts."

He added: "Despite the excellent protective value of the coronavirus vaccine in preventing hospitalization and death, there is still a possibility a fully-vaccinated individual could acquire infection in their nose and throat, mild symptoms, or the ability to transmit the coronavirus infection to others."

Monahan further warned about the possibility of "long COVID" and cited the fact that some local governments have either mandated masks or recommended them already in light of the delta variant. 

House Speaker Nancy Pelosi, D-Calif., on Wednesday said that Monahan made the decision to bring back the mandate on his own and she was not involved.

The CDC's decision Tuesday was highly controversial, with many alleging that it will undermine faith in vaccines – which prevent nearly all severe infection, hospitalization and death from COVID-19 – just as the U.S. is trying to increase vaccination rates among hesitant populations. 

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Others worried that local and state governments would use the new recommendation from the CDC as a reason to implement new mandates, requiring vaccinated people to wear masks indoors even if they are healthy, confident that their vaccines work and in a situation where they are comfortable not masking. 

But Walensky and the White House defended the new guidance as grounded in science and aimed at protecting Americans. 

"Information on the delta variant from several states and other countries indicate that in rare occasions, some vaccinated people infected with the delta variant after vaccination may be contagious and spread the virus to others," Walensky told reporters Tuesday. "This new science is worrisome and unfortunately warrants an update to our recommendations."

Fox News' Chad Pergram, Jason Donner and Peter Doocy contributed to this report.