US health officials said the contagious Delta variant has rapidly become the most dominant strain of coronavirus in the country amid warnings that the full reopening of the US economy may need to be curtailed by authorities.

Rochelle Walensky, director of the Centers for Disease Control and Prevention, told a Senate health committee hearing on Tuesday that her agency now estimates that Delta accounts for 83 per cent of sequenced cases.

“This is a dramatic increase, up from 50 per cent for the week of July 3,” Walensky said. In some parts of the country, the percentage was even higher, particularly in areas with low vaccination rates, she added.

The Delta variant, first discovered in India, has rapidly spread through a number of countries, including those with high vaccination rates, including the US and UK, alarming health officials and raising fears that restrictions will need to be reimposed to curb the spread.

At the weekend, Dr Vivek Murthy, the US surgeon-general, warned that more US cities might follow the lead of Los Angeles and reintroduce mask mandates for indoor settings, even among people who are fully vaccinated, to curb the spread of the Delta variant.

Anthony Fauci, director of the US National Institute of Allergy and Infectious Diseases, said during an interview on CBS This Morning, that the CDC was reviewing its guidelines for mask use by children in schools following a recommendation from the American Academy of Pediatrics that all students and staff aged 2 years and older wear masks, regardless of vaccination status.

Over the past week, the US had averaged 239 deaths a day from Covid-19, an increase of almost 48 per cent over the previous week, Walensky said.

She previously warned that the US was experiencing a “pandemic of the unvaccinated” with cases rising quickly again, particularly among those who are unable or unwilling to get vaccinated despite plentiful supply.

More than 161m Americans are fully vaccinated, representing about 48.6 per cent of the US population, according to CDC data. In almost two-thirds of counties in the US, less than 40 per cent of people had been vaccinated, Walensky said on Tuesday.

The Biden administration has expressed concern that vaccine hesitancy is being fuelled by misinformation shared on social media platforms and has engaged in a public spat with Facebook.

“The message from the CDC remains clear: the best way to prevent the spread of Covid-19 variants is to prevent the spread of disease, and vaccination is the most powerful tool we have,” Walensky said.

Fauci said the vaccines authorised for use in the US are still highly effective against the “formidable” Delta variant.

“We have the tools to end this epidemic it is up to us to utilise those tools to their maximum,” he said. He said that studies were being done to determine whether vaccinated people would need booster shots “to increase durability of protection”.

Kate Bedingfield, the White House director of communications, told CNBC on Tuesday that she thought social media companies should be held accountable for vaccine misinformation published on their platforms. She said the White House was “reviewing” Section 230 of the Communications Decency Act, which grants social media companies legal immunity for content which users publish on their platforms.

Cases have also been reported among fully vaccinated individuals. A spokesperson in the office of Nancy Pelosi, the speaker of the US House of Representatives, on Tuesday was confirmed to have tested positive for Covid-19 after meeting Democratic members of the Texas state legislature last week who later tested positive for the virus. 

Additional reporting by Kiran Stacey and James Politi in Washington



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