As of Tuesday, 65.4 per cent of over-18s had received one or more doses of the Pfizer, Moderna or Johnson & Johnson shots.

Seventy per cent of over-30s nationally have received one or more shots, and 70 per cent of over-27s were expected to be partially vaccinated by July 4, the day the US commemorates its Declaration of Independence.

Nationwide, the vaccination rate has been declining since April when it hit a peak average of 3.4 million daily shots. The latest average is around 850,000 daily shots.

Delta coronavirus variant spreading in unvaccinated pockets of US

Adult immunisation rates vary greatly by state. The Northeast has some of the highest uptake, with Vermont leading the charge at 84 per cent partially vaccinated and 75 per cent fully vaccinated.

The bottom of the table is dominated by states in the South, with Mississippi covering just 45 per cent of adults with one shot and 37 per cent with two.

According to nationally representative surveys carried out by the Kaiser Family Foundation, unvaccinated adults are significantly younger, less educated, more likely to be Republicans, people of colour and uninsured.

Even so, the country is registering its fewest cases since the pandemic began, as a result of current levels of vaccination, people who have gained a degree of immunity from past infection, as well as seasonality associated with the virus.

The pathogen survives longer in cold and dry weather, and transmits more readily indoors.

The latest seven-day-average is around 10,000 new infections – just three per 100,000 people. Daily deaths are at around 270 on average, or 0.1 per 100,000.

“Instead of just small backyard gatherings, America is getting ready for a truly historic Fourth of July, with large celebrations planned in communities across the country,” said Zients.

He added the White House was preparing for an outdoor party with 1,000 military personnel and frontline workers to celebrate the upcoming holiday.

The Centres for Disease Control and Prevention (CDC) lifted its masking recommendations for vaccinated people in May – making the US among the first countries to do so – and the policy appears to have been vindicated so far.

WHO switches to Greek alphabet for variant names to avoid stigma

Still, officials remain deeply concerned about unvaccinated people who remain vulnerable.

“Nearly every death, especially among adults, due to Covid-19, is at this point entirely preventable,” said CDC director Rochelle Walensky, emphasising that vaccines were close to 100 per cent effective against death and against severe cases of Covid-19.

The July 4 target of 70 per cent “is not the goal line, nor is it the end game,” added Anthony Fauci, Biden’s chief medical adviser on the pandemic.

“The end game is to go well beyond that, beyond July 4 into the summer and beyond, with the ultimate goal of crushing the outbreak completely in the United States.”

Epidemiologists are also worried about the rise of the Delta variant, which is more transmissible than past strains and badly hit India, Britain and other countries. It now accounts for at least 20 per cent of US cases.

Current vaccines remain highly effective against the variant if a person is fully vaccinated, but there are fears that undervaccinated parts of the country could incubate fresh surges.

Fauci said these spikes would likely remain localised and not close to the same magnitude of the three major waves the United States has experienced since the spring of 2020.

Additional reporting by Reuters