Instead he has rolled out a new financial incentive for those who have yet to get immunised.

The incentive comes in the form of a tax credit allowing businesses with fewer than 500 employees to fully offset the cost of providing upward of 80 hours, or 10 workdays, of paid time off for employees who need it to get the vaccine, Biden said.

The extensive span in paid time off is meant to give workers time to recover from any vaccination-related side effects without having to worry about missing a pay cheque, he said.

"One concern I've heard from so many Americans is that they can't afford to take a day off to get vaccinated," Biden said at the White House on Wednesday.

"No working American should lose a single dollar from their pay cheque because they chose to fulfil their patriotic duty of getting vaccinated.

"We're calling on every employer, large and small, in every state: give employees the time off they need with pay to get vaccinated."

The push for pumping out more vaccine came on the same day as the US reached Biden's goal of administering 200 million shots within his first 100 days in office.

"It's an incredible achievement for the nation" said Biden, who at first anticipated the US would inject 100 million shots within his first 100 days in office, but raised the stakes after that goal was met faster than expected.

Wednesday marked Biden's 91st day in office.

The 200 million milestone translates to about half of the American population getting at least one shot.

More than one-third of all Americans are now fully vaccinated, according to data from the Centers for Disease Control and Prevention.

But Biden said the US is now entering a challenging "new phase" in the race to vaccinate the country out of the pandemic that has killed more than 560,000 Americans.

Even though every state now allows anyone over 16 to get vaccinated, only about 43 per cent of working Americans have received at least one shot, according to the CDC.

The low rate in that demographic appears to in part be a product of persistent vaccine hesitancy, especially among younger people, some communities of colour and political conservatives.

The government is pouring hundreds of millions of dollars into outreach efforts aimed at counteracting such vaccine scepticism.

Public health officials say about 80 per cent of the population needs to be fully vaccinated for the US to have a chance at achieving herd immunity.

Biden promised Americans last month that the country should be back to something that resembles normal by July 4.

Australian Associated Press