The top US health agency on Tuesday changed its guidance for vaccinated people, recommending the wearing of face masks in public indoor spaces if they are in an area of high transmission and low vaccination rates.
“This pandemic continues to pose a serious threat to the health of all Americans,” Centres for Disease Control and Prevention (CDC) Director Rochelle Walensky said.
The country is seeing a new surge of infections and hospital admissions, mostly among the unvaccinated population, due to the highly transmissible Delta variant that is now dominant nationwide.
Dr Walensky said the variant is “showing every day its willingness to outsmart us".
More than 42,000 people are testing positive for coronavirus daily, the CDC's seven-day moving average showed.
Dr Walensky cited new research that found “some vaccinated people infected with the Delta variant may be contagious".
The new guidance says fully vaccinated people should wear a mask indoors in places where there is “high” or “substantial” transmission of Covid-19.
The CDC deems “high transmission” to be when there are more than 100 positive cases per 100,000 people in a population in the last seven days — and there are at least 16 states experiencing that level of infection.
The next tier down is “substantial transmission”, in which between 50-99 cases are recorded for every 100,000 people, and the CDC reports that 14 states are currently at that level.
“The vast majority of transmission, severe disease, hospitalisation and death is almost exclusively happening among unvaccinated people,” Dr Walensky said.
The new guidance appeared to be a partial reversal of the guidance the CDC issued this May, in which it ruled that fully vaccinated people could do most things without a face mask, after recommending people wear masks and maintain three metres of distance throughout the pandemic.
The previous guidance allowed states, cities, offices and businesses to lift mask mandates for fully vaccinated people and reopen public spaces, though this had the effect of complicating mask and social-distancing enforcement for unvaccinated people.
It is not clear how states, cities and businesses will react to the new recommendations, as the agency does not impose or enforce local laws.
However, the guidance issued in May was given at a time of high vaccination rates and fewer recorded cases. In the months following, immunisations have slowed and infections are on the rise due to the Delta variant.
Some US cities like Los Angeles and St Louis have recently reissued indoor mask mandates regardless of vaccination status.
The CDC on Tuesday also recommended that everyone in US schools wear a mask, as large numbers of young people remain unvaccinated. Children under the age of 12 are not yet eligible to receive a Covid-19 vaccine.
Only 49.1 per cent of the US population is fully vaccinated, but that statistic increases to 57.5 per cent in the case of the vaccine-eligible population over the age of 12.
US President Joe Biden is facing some backlash as he failed to reach his goal of having 70 per cent of the country's adult population receive at least one dose of vaccine by July 4 — though the goal is in now reach, with that figure currently at 69 per cent.
Mr Biden said in a statement on Tuesday that he will “lay out the next steps in our effort to get more Americans vaccinated” on Thursday.
“By following the science and by doing our part by getting vaccinated, America can beat Covid,” he said.
Mr Biden also said that requiring all federal workers to be vaccinated against Covid-19 was now "under consideration". The move would make the government one of the nation's largest employers to make such a move.
His comments come after the Department of Veterans Affairs became the first US federal agency to require its healthcare employees be vaccinated. California and New York City also issued similar requirements for employees to be vaccinated by a certain date or face weekly Covid-19 tests.
The US is one of the few countries to have plenty of vaccine supply. Despite this, its population ranks high globally for vaccine hesitancy.
Dr Walensky said the current situation “could have been avoided” if more people had been vaccinated.
The worry is that, if the virus continues to spread unabated, a more serious variant could develop.
“The big concern is that the next variant that might emerge, just a few mutations potentially away, could potentially evade our vaccine,” Dr Walensky said.
More than 611,000 people in the US have died from Covid-19, Johns Hopkins University reported, the highest official count recorded in the world.