The Biden administration admitted that it would miss the US president’s target of vaccinating 70 per cent of American adults by July 4, calling it an “aspirational goal”.

Jeffrey Zients, White House Covid-19 response co-ordinator, said 70 per cent of Americans aged over 30 had received at least one dose of a coronavirus vaccine. However, he said it would take “a few extra weeks” to reach Joe Biden’s target of distributing at least one jab to 70 per cent of all adults by July 4.

Biden set the goal early last month, but the country’s vaccine rollout has slowed after a blistering start as health officials struggle against vaccine hesitancy.

“We have succeeded beyond our highest expectations,” Zients said at a press briefing on Tuesday. “Instead of just small backyard gatherings, America is getting ready for a truly historic Fourth of July with large celebrations planned across the country.”

Sixteen states and the District of Columbia have already hit the president’s goal. Among US adults aged 18 and older, 65 per cent have received at least one vaccine dose, according to figures from the US Centers for Disease Control and Prevention. More than 150m Americans are fully vaccinated.

After targeting older people and essential workers in the first wave of vaccinations, the focus has turned to vaccinating young adults, which Zients said “have felt like Covid-19 is not something that impacts them”. He said “the country has more work to do” to encourage vaccine uptake among those aged 18 to 26.

The Biden administration now expects that 70 per cent of Americans aged 27 and older will be immunised by the end of the July 4 weekend, Zients said, adding that Biden’s second target of reaching 160m fully vaccinated adults would be met “no later than mid-July”.

State officials across the country have created innovative campaigns to encourage vaccine uptake such as offering free beer and guns, as well as million-dollar raffles, but the speed of the immunisation programme has faltered in the past few weeks.

The need to inoculate holdouts has taken on fresh urgency as the Delta variant, which first emerged in India, spreads across the US. It has swept across the UK and been blamed for rising cases in other parts of the world.

Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases, warned about the threat of Delta to the US recovery and unvaccinated people, saying “its transmissibility is unquestionably greater” than the original strain of Covid-19 and the Alpha variant that was first detected in the UK.