The Centers for Disease Control and Prevention quietly removed guidance for phasing out masks and other COVID-19 mitigation efforts in schools, cached versions of the agency’s website show. 

The CDC made the changes when it updated its guidance on universal indoor masking for students, staff, teachers and visitors to K-12 schools, regardless of their vaccination status, on Aug. 5. 

"We believe that our state, as well as teachers unions, probably had an influence over this change," Jonathan Zachreson, an advocate for fully reopening California schools, told Fox News. "It's basically mask indefinitely in schools forever, and there is no off-ramps. So it's really disappointing to see that." 

Teachers unions have previously influenced changes to the CDC’s school-related guidance, reports have shown.

COVID-19 VACCINE AND TESTING MANDATE GETS SUPPORT FROM LARGEST US TEACHERS UNION

CDC spokeswoman Jade Fulce told Fox News that guidance is "always being revised based on the current epidemiology" and that increases in the delta variant and low vaccination coverage in some communities led to the changes. 

But the CDC’s previous guidance only suggested transitioning out of COVID-19 precautions, including masking students, as cases decreased. So mitigation efforts would remain in the case of a surge or a high infection rate. 

The language in the agency’s "Guidance for Covid-19 Prevention in K-12 Schools," intended to help school administrators and local health officials transition from prevention strategies as cases moved to lower levels, was removed after Aug. 4, a cached version of the webpage shows.

A note on the guidance states that it was updated to recommend indoor masking for everyone, but doesn't mention that any language was removed.

The CDC guidance on Aug. 4 indicated that it was intended to help school administrators and local health officials determine which prevention strategies to use and how to transition "out of COVID-19 pandemic precautions as community transmission of COVID-19 reaches low levels or stops." 

That portion of the guidance is now gone. 

CDC guidance on Aug. 4

CDC guidance now 

Initially, the CDC's guidance included metrics that could help schools decide which mitigation strategies to use. But those metrics were removed.

The guidance instead states that schools should determine what "additional" strategies should be used as they monitor transmission levels. 

CDC guidance on Aug. 4

CDC guidance now

The CDC’s guidance on Aug. 4 recommended that schools continue masking and distancing, but noted that they if they decide to scale back any prevention strategies, "they should remove them one at a time and monitor closely." 

That language was removed entirely. 

CDC guidance on Aug. 4

CDC guidance now

The CDC had also recommended that school masking policies do not conflict with any state laws or regulations. This portion no longer appears on the site.

CDC guidance on Aug. 4

CDC guidance now

"They removed all off-ramps," Zachreson, who leads the group Reopen California Schools, told Fox News. "So other mitigations, removing masking – even if COVID prevalence goes down to zero or vaccination rates increase – the CDC guidelines do not recommend removing these protocols for schools like they previously did." 

"I know that goes against what many infectious disease experts are telling us, especially here in California," Zachreson continued. "This also comes as there was a conflict with … our state's health department's recommendations on masking." 

Zachreson's comments follow reports showing teachers unions influencing the CDC on its earlier school guidance. 

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In May, the New York Post reported on emails the watchdog group Americans for Public Trust had received showing coordination between the Biden White House and teachers unions on school reopenings. 

The Post's report showed that in at least two instances, suggestions that the American Federation of Teachers offered were "adopted nearly verbatim" in the final text of a CDC document on reopening guidance released in February. 

Earlier in September, Fox News reported on emails obtained by Americans for Public Trust showing that the CDC tightened its masking guidelines in May after receiving threats from a teachers union.