Answering growing frustration over vaccine shortages, President Joe Biden has announced the US is surging deliveries to hard-pressed states over the next three weeks and expects to provide enough doses to vaccinate 300 million Americans by the end of summer or early autumn.
Biden, calling the push a "wartime effort", said the administration was working to buy an additional 100 million doses of each of the two approved coronavirus vaccines.
He acknowledged that states in recent weeks had been left guessing how much vaccine they will have from one week to the next.
Shortages have been so severe that some vaccination sites around the US had to cancel tens of thousands of appointments with people seeking their first shot.
"This is unacceptable," Biden said on Tuesday. "Lives are at stake."
He promised a roughly 16 per cent boost in deliveries to states during the next three weeks.
The administration plans to buy another 100 million doses each from drug makers Pfizer and Moderna to ensure it has enough vaccine for the long term.
Even more vaccine could be available if federal scientists approve a single-dose shot from Johnson & Johnson, which is expected to seek emergency authorisation in the coming weeks.
The Centers for Disease Control and Prevention reported that the government plans to make about 10.1 million first and second doses available next week, up from this week's allotment of 8.6 million.
The figures represent doses of both the Pfizer and Moderna vaccines. It was not immediately clear how long the surge of doses could be sustained.
Governors and top health officials have been increasingly raising the alarm about inadequate supplies and the need for earlier and more reliable estimates of how much vaccine is on the way so that they can plan.
Biden made management of the pandemic a core issue in his presidential election campaign, but in its early days, the administration has sent mixed messages about when exactly the vaccines will be fully administered.
On Monday, Biden said he believed it was possible to have 150 million doses of the vaccine administered in his first 100 days in office, an aspiration his press secretary Jen Psaki said was not an official adjustment of the current target of 100 million doses over that same period.
The outbreak has killed more than 420,000 Americans.
Australian Associated Press