WASHINGTON - The US Centers for Disease Control and Prevention (CDC) in a reversal on Tuesday (July 27) recommended that fully vaccinated people wear masks indoors in areas with significant or high spread of the coronavirus that causes Covid-19.
At present, with the exception of the Northeast and parts of the Upper Midwest, much of the United States including Washington DC falls into that category. The White House and Congress will now again require masks to be worn indoors, too.
"The Delta variant is showing every day its willingness to outsmart us," CDC director Dr Rochelle Walensky said.
"When we examine the rare breakthrough infections and we look at the amount of virus in those people, it is pretty similar to the amount of virus in unvaccinated people," she said.
Thus, vaccinated people with breakthrough cases of the Delta variant might be able to spread it.
Some Republicans who have resisted mask and vaccine mandates reacted sharply. The CDC's decision was "conjured up by liberal government officials who want to continue to live in a perpetual pandemic state," House Minority Leader Kevin McCarthy tweeted.
But the decision was not taken lightly, Dr Walensky said.
The US is dealing with a new wave of hospitalisations on the back of a stalling vaccination rate and the rise of the Delta variant.
"Breakthrough" cases - of people fully vaccinated yet contracting Covid-19 - are also causing concern.
An outbreak in Provincetown, Massachusetts, has led to at least 132 people infected since July 1. Most of those affected were vaccinated.
Still, the vast majority of those hospitalised are the unvaccinated. Even children have had to be hospitalised. The surge is expected to accelerate, peaking in mid-October, with daily deaths potentially more than triple what they are now.
This has parents worried, as schools will reopen in September after the summer break. Children will now have to wear masks in school.
"What's going on in the country with the virus is matching our most pessimistic scenarios," University of North Carolina epidemiologist Justin Lessler told National Public Radio.
Dr Lessler helps run the Covid-19 Scenario Modeling Hub, a consortium working in consultation with the CDC.
"We might be seeing synergistic effects of people becoming less cautious in addition to the impacts of the Delta variant," Dr Lessler warned.
Currently, about half the US population is vaccinated, but the rate has significantly slowed as vaccine scepticism - fanned by right-wing conservative media and disinformation on social media - remains strong in pockets around the country, especially in heavily Republican states.
In the most likely scenario, Dr Lessler said, with the vaccination rate reaching 70 per cent, by the peak in mid-October there would be around 60,000 cases and around 850 deaths each day.
According to the New York Times' Covid tracker, nationwide on July 27 there were 290 deaths from the coronavirus.
Since the weekend, a string of local governments and organisations have scrambled to impose vaccination and mask mandates previously thought politically and socially controversial.
On Monday, New York City and the state of California announced that they would require hundreds of thousands of government workers to get vaccinated or face weekly testing. And the city of St Louis, in Missouri, has imposed an indoor mask mandate.
From Thursday, hundreds of bars in San Francisco will begin requiring proof of vaccination or negative Covid-19 tests from customers who want to be indoors. The measure comes on the back of a rise in cases among fully vaccinated bar workers.
"We believe we are obligated to protect our workers and their families and to offer safe space for customers to relax and socialise," the San Francisco Bar Owner Alliance said in a statement.
On Monday, the Department of Veterans Affairs, which with 2.1 million workers is the federal government's largest employee, became the first federal agency to require Covid-19 vaccinations.
Fifty-seven medical organisations, including the American Medical Association, the American Nurses Association and the American Academy of Paediatrics, on Monday released a joint statement urging such vaccine requirements.
On Tuesday, Washington Post publisher Fred Ryan announced that all employees of the paper need to demonstrate proof of full Covid-19 vaccination as a condition of employment.
"The floodgates have opened," Brown University School of Public Health dean Ashish Jha tweeted.
"More and more organisations are realising that there is no way back to safe offices, safe hospitals, and safe universities without vaccinations."
Dr Aaron Wendelboe, associate professor of biostatistics and epidemiology at the University of Oklahoma's Health Sciences Centre, told local station KFOR: "We don't have to experience the hospitalisation, we don't to have to shut down society."
"We can just utilise what's essentially right in front of us, the vaccine and masks."