WASHINGTON (REUTERS) - More than half of all adults in the United States have been fully vaccinated against Covid-19, the White House said on Tuesday (May 25), roughly six weeks before US President Joe Biden's July 4 goal of 70 per cent of the adult population receiving at least one shot.

The halfway mark comes as federal, state and local leaders press ahead with delivering Covid-19 shots to people who have not yet received them, while also battling vaccine hesitancy, fears and misinformation.

"Now, with another week left in May, half of all US adults are fully vaccinated," White House senior Covid-19 adviser Andy Slavitt tweeted overnight.

New coronavirus infections nationwide have settled into a sustained decline as more people become vaccinated.

US Centres for Disease Control and Prevention (CDC) director Rochelle Walensky said on Tuesday the seven-day average case count in the United States is now below 23,000 per day, down about 25 per cent from the prior week.

"I remain cautious but hopeful they will continue to trend downward," Walensky said, adding that people who are not yet vaccinated should still follow federal guidance on social distancing and mask wearing during the upcoming Memorial Day weekend.

The seven-day average of new infections at 22,877 on Sunday was the lowest since June and less than one-tenth of its peak of more than 250,000 following the Christmas and New Year holidays, according to CDC data.

People as young as 12 can now receive the Pfizer/BioNTech Covid-19 shot, while Moderna said on Tuesday its vaccine was also effective in adolescents, opening the door to a likely second US-authorised vaccine for youths later this year.

A growing number of localities have announced further reopening efforts as the United States heads into its unofficial summer kick-off with the Memorial Day holiday this weekend.

US travel is on the rise and, while masks are still required for interstate public transportation, many local mandates have been lifted.