Joe Biden vowed on Tuesday to ramp up vaccination programs so that most of the US population is inoculated by the end of summer or early fall.

“This will be enough vaccine to fully vaccinate 300m Americans by the end of the summer,” the US president said on Tuesday afternoon, later adding “end of summer, beginning of the fall”, in a briefing at the White House.

The new administration will increase vaccine supplies to states, exercise an option to buy a total of 200m more vaccine doses from Pfizer and Moderna and will give states more lead time on the amount of vaccine it will deliver.

The administration’s immediate plan is to accelerate vaccine distribution to deliver roughly 1.4m shots a day and 10m doses a week for the next three weeks, as part of the White House’s earlier-stated ambition to vaccinate 100 million people in 100 days.

“This will be one of the most difficult operational challenges we’ve ever undertaken,” said Joe Biden on Tuesday, announcing the plans. But, he added, “Help is on the way”.

He indicated that the vaccination program he inherited from the Trump administration was not in adequate order.

“When we arrived, the vaccine program was in worse shape than we expected or anticipated,” Biden said.

He added: “Until now, we’ve had to guess how much vaccine to expect for the next week, and that’s what the [state] governors had to do. This is unacceptable.”

The new purchase order is expected to allow the government to vaccinate 300 million people with a two-dose regimen of either the Pfizer or Moderna vaccine, a senior administration official said earlier.

The official said there were two “constraining factors”, for delivering vaccines quickly: supply and distribution. The official said the White House was working to increase capacity for both, by purchasing more vaccine, raw supplies and setting up federal vaccination sites.

“This is a wartime undertaking, it’s not hyperbole,” said Biden.