People vaccinated against Covid-19 in high-risk parts of the United States should resume wearing masks indoors, the country's top health authority has said.
This is a major reversal in guidance underpinning the country's struggle to suppress the Delta variant.
President Joe Biden said the announcement showed that the US needs to "do better" on vaccinations, adding that a vaccine mandate for the country's more than two million federal workers was now "under consideration".
Centers for Disease Control and Prevention (CDC) director Rochelle Walensky cited new data that shows rare breakthrough cases involving Delta have an increased the risk of onward transmission.
"In areas with substantial and high transmission, CDC recommends fully vaccinated people wear masks in public indoor settings," she said.
As recently as last week, the CDC had defended its surprise decision in May that vaccinated people did not have to wear masks indoors in most circumstances.
In another setback, the White House yesterday also ordered all its staff to mask up again due to local transmission rates in Washington.
According to the latest CDC data, 63% of the country's more than 3,200 counties are experiencing substantial or high transmission.
Substantial is defined as being between 50 to 100 daily cases per 100,000 people over seven days, while high is defined as more than 100 daily cases per 100,000 over seven days.
President Biden said he would lay out new steps tomorrow to overcome the lag in vaccinations after the country's strong start to its programme.
In a separate address to the US intelligence community, when asked about a possible vaccine mandate for US federal workers, Mr Biden replied: "That's under consideration right now."
On Monday, the Veterans Affairs department said it would require its frontline health workers, some 115,000 people, to get the shot, becoming the first federal agency to institute the requirement.
Ms Walensky stressed that so-called "breakthrough" cases among people who are vaccinated remain rare - shots reduce the risk of symptomatic disease seven-fold, and hospitalisations and deaths by a factor of twenty.
However, new CDC research showed that when a vaccinated person does become infected, their viral load is similar to an unvaccinated person.
Former president Donald Trump, widely accused of mishandling the virus while in office, slammed the latest recommendations.
"We won't go back. We won't mask our children," he said in a statement. "Brave Americans learned how to safely and responsibly live and fight back."
Infection numbers in the United States are now swelling, thanks to the Delta variant, which accounts for around 90% of cases.
The latest seven-day average of daily cases is more than 56,000, similar to levels last seen in April. In total, more than 610,000 people have died of Covid-19 in the United States.
Around 49% of the US population is fully vaccinated, but the vaccination rate is heavily skewed towards politically liberal parts of the country.
New York orders frontline health care workers to get vaccinated
New York state will require all "public-facing" health care workers to get vaccinated from September, Governor Andrew Cuomo has announced, as the United States struggles to suppress the Delta variant.
Mr Cuomo said all of the state's tens of thousands of employees would have to show proof of vaccination or face weekly tests from 6 September.
But he announced stricter measures for frontline workers at state-run hospitals, saying they would not have the option of being tested.
"New York state will require patient-facing healthcare workers at state hospitals to get vaccinated to help keep both patients and workers safe," Mr Cuomo said.
Earlier this week, California and New York City announced that official workers would need to get vaccinated or take weekly tests.
California's order will apply to almost 240,000 state workers and hundreds of thousands more private-sector health workers, with full compliance required by 21 August.
New York City's mandate will go into effect from 13 September and apply to more than 300,000 city personnel, including police, fire fighters and teachers.
The US Department of Veterans Affairs became the first federal agency to impose a mandate when it announced on Monday that it would require more than 100,000 health care personnel to get vaccinated.
Some 59% of all eligible New York City residents have received at least one vaccine shot.
Mayor Bill de Blasio announced a new incentive: $100 when you receive your first dose.