WASHINGTON (AP) — The White House is strongly considering requiring federal employees to show proof they’ve been vaccinated against the coronavirus or otherwise submit to regular testing and wear a mask — a potentially major shift in policy that reflects growing concerns about the spread of the more infectious delta variant.

The possible vaccine mandate for federal employees — regardless of the rate of transmission in their area — is one option under consideration by the Biden administration, according to a person familiar with the plans who spoke on condition of anonymity to discuss deliberations that have yet to be made public. The White House is expected to announce its final decision after completing a policy review this week.

According to an analysis from the federal Office of Management and Budget, in 2020 there were more than 4.2 million federal workers nationwide, including those in the military.

President Joe Biden suggested Tuesday that expanding that mandate to the entire federal workforce was “under consideration,” but offered no further details. The Department of Veterans Affairs on Monday became the first federal agency to require vaccinations, for its health workers.

The broader requirement under consideration would be the most significant shift by the Biden administration this week as the White House grapples with a surge in coronavirus cases and hospitalizations nationwide driven by the spread of the delta variant and breakthrough infections among vaccinated Americans.

On Tuesday, the Centers for Disease Control and Prevention reversed its masking guidelines and said that all Americans living in areas with substantial or high coronavirus transmission rates should wear masks indoors, regardless of their vaccination status.

And just like that, masks were back at the White House.

By Tuesday afternoon, when the latest CDC data found that Washington, D.C. is facing substantial rates of transmission, White House staff were asked to begin wearing masks indoors starting Wednesday. Press were asked to follow suit, and those staff and reporters remaining in the White House were already masking up.

An aide for Vice President Kamala Harris passed out masks to the reporters covering her events earlier that day, asking them to put them on before walking in to her meeting with Native American leaders on voting rights.

Biden dismissed concerns that the new masking guidance from the CDC could create confusion among Americans, saying those who remain unvaccinated are the ones who are “sowing enormous confusion.”

“The more we learn, the more we learn about this virus and the delta variation, the more we have to be worried and concerned. And there’s only one thing we know for sure — if those other 100 million people got vaccinated, we’d be in a very different world,” he told reporters after speaking to intelligence community employees at the Office of the Director of National Intelligence on Tuesday.

But the whiplash on masking and vaccinations — just the day before, White House press secretary Jen Psaki had avoided questions over why the administration had yet to require vaccines for federal workers — reflects the uncertainty surrounding the coronavirus.

Various state and local governments, private companies, hospital administrators and universities across the nation have reverted to indoor mask mandates and instituted vaccine mandates in recent months, but just 60% of American adults have been completely vaccinated, and the latest wave of the coronavirus is hitting those communities with low vaccination rates particularly hard. The nation is averaging more than 57,000 cases a day and 24,000 COVID-19 hospitalizations.

But the Biden administration had thus far avoided embracing a vaccine mandate for its own employees — in part because officials are wary of further politicizing an already fraught issue by coming down too hard on the side of vaccine mandates.

Psaki acknowledged Tuesday that administration officials are aware of the risk that Biden’s support for vaccine mandates could harden opposition to vaccines among his detractors.

“The president certainly recognizes that he is not always the right voice to every community about the benefits of getting vaccinated, which is why we have invested as much as we have in local voices and empowering local, trusted voices,” she said.