US federal health officials revised their guidance to recommend indoor masking for the vaccinated in areas where the level of transmission of the coronavirus is substantial or high.

That includes Washington DC, where updated data from the Centers for Disease Control and Prevention on Tuesday found that transmission rates were substantial.

Starting on Wednesday, the White House will require all of its employees to again wear masks indoors. White House reporters were asked to follow those guidelines and mask up indoors as well..

And President Joe Biden hinted there may be more changes to come, with a surge in the highly transmissible delta variant of the coronavirus driving spiking infection rates nationwide.

After the Department of Veterans Affairs announced on Monday it would require its health care workers to get vaccinated, Biden suggested that expanding that mandate to the entire federal workforce was "under consideration".

Biden also dismissed concerns 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.

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 amid a rise in breakthrough infections among vaccinated Americans and an increase in COVID-related hospitalisations as the delta variant continues to spread.

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 per cent 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 hospitalisations.

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

Psaki acknowledged on 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 recognises 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.

Australian Associated Press