More than half of all American adults have been vaccinated against Covid-19, the White House said, roughly six weeks before US President Biden's July 4 goal of a 70 per cent inoculation rate.

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

The number of new US infections fell to a seven-day average of 22,877 on Sunday, the lowest since June and less than one-tenth of its post-holiday peak of more than 250,000, according to Centres for Disease Control and Prevention data.