The United States is making a push to inoculate more younger adults, amid concerns that demand for Covid-19 jabs is slowing as the spread of the Delta variant picks up speed.

Younger adults remain under-vaccinated compared with older adults.

The highly contagious and more dangerous Delta strain now makes up 20.6 per cent of new infections in the United States, more than double the 9.6 per cent level early this month, according to the Centres for Disease Control and Prevention (CDC) on Tuesday.

Although vaccines have proven effective against the variant, public health officials worry that its rapid spread could threaten efforts to overcome the pandemic.

"Similar to the situation in the United Kingdom, the Delta variant is currently the greatest threat in the US to our attempt to eliminate Covid-19," top infectious disease expert Anthony Fauci said at a press briefing.

He noted that variant cases were doubling every two weeks.

The White House acknowledged on Tuesday that the US will narrowly miss President Joe Biden's goal of getting 70 per cent of adults at least partially vaccinated by July 4, Independence Day.

It is now at 65.5 per cent.

But those aged 30 and older have already achieved that 70 per cent threshold, and those 27 and older are on track to reach that target by July 4, said White House coronavirus response coordinator Jeff Zients.

The nation needs "a few extra weeks" to get Americans between 18 and 26 similarly vaccinated, he added.

But reaching that threshold "is not the goal line, nor is it the endgame", said Dr Fauci.

"The goal line is to completely crush this outbreak."

The vaccination programme has gained momentum since Mr Biden took office in January, and the pace of deaths and infections has slowed.

The US recorded an average of 11,000 new cases daily on Monday for the first time since cases started climbing in March last year.

But the inoculation drive has slowed, particularly in the South and Mid-west, with incentives like state lotteries, college scholarships and other freebies no longer having the pull they used to.

America's youngest adults have been particularly hard to persuade.

The CDC said on Monday that they have the lowest vaccination coverage, with just 38.3 per cent of those aged 18 to 29, compared with 80 per cent of those older than 65.

"If the current rate of vaccination continues through August, coverage among young adults will remain substantially lower than among older adults," the CDC said.

While the weekly rates of people above 65 getting their first shot peaked at 8.2 per cent, this level was never reached by younger adults and has been decreasing.

"Where the country has more work to do is particularly with 18-to 26-year-olds," said Mr Zients.

"Many younger Americans have felt like Covid-19 is not something that impacts them."

The White House has organised Instagram Live sessions and a YouTube townhall with influencers, and even taken out advertisements on video gaming platforms urging people to get jabs.

Wrote Mr Biden on Twitter: "When we took office, 3,000 Americans were dying every day. Now, hospitalisations have dropped by more than 90 per cent.

"I'm urging young people to carry us across the finish line by getting vaccinated today."