Updated at 5:46 p.m. ET

President Biden announced Tuesday that his administration is working to purchase an additional 200 million doses of the two COVID-19 vaccines that have been authorized for emergency use, with the goal, the White House says, of having enough vaccine supply for the entire adult U.S. population by the end of the summer.

He also announced steps to increase vaccine doses going to state and local governments over the next three weeks, and to provide them more clarity going forward about how much supply they should expect.

Longer term, Biden said his administration plans to buy an additional 100 million doses each from both Moderna and Pfizer, which has a vaccine with its German counterpart, BioNTech.

"This increases the total vaccine order for the U.S. by 50%, from 400 million to 600 million with these additional doses expected to deliver this summer," the White House said in a fact sheet. The Trump administration had secured contracts with the two companies for the already-pledged 400 million doses.

Both the Moderna and Pfizer vaccines require a two-dose regimen per person. The Moderna vaccine is currently authorized for people 18 and older, while the Pfizer vaccine right now is recommended for people 16 and up.

An NPR analysis last week found that the two drugmakers needed to increase their pace of production to meet their original promises of 100 million doses apiece delivered to the United States by the end of March. Moderna said it was on track to hit its mark, while Pfizer didn't respond to NPR's request for comment.

Biden's announcement about the expected additional supply comes nearly a week into his presidency, and as he has repeated that the vaccine rollout — to help alleviate the coronavirus pandemic and to put unemployed Americans back to work — is his top priority. More than 420,000 Americans are confirmed to have died from COVID-19 — a figure that's likely a severe undercount.

Biden has said his goal is to get 100 million shots into arms in the first 100 days of his administration, a figure that was in line with earlier trends and has been called too low by many experts. Speaking Monday, Biden threw out a higher figure he'd like to see: 150 million shots.

The allocation and delivery of vaccines — now crossing two administrations — has been troubled. Some 20 million more doses have been distributed by the federal government than have been administered to people, according to the latest tally from the Centers for Disease Control and Prevention.

In a call with senior administration officials before Biden's remarks, one official noted two constraining factors on the inoculation effort: supply of vaccines and an ability by states and localities to administer vaccines quickly.

Over the next three weeks, the Biden administration said it will boost supply to states, tribes and territories from a current 8.6 million doses a week to a minimum of 10 million doses per week.

And responding to complaints from state and local officials about how much supply they were due to receive, the administration is also pledging more transparency.

"The Department of Health and Human Services will provide allocation estimates for the upcoming three weeks as opposed to the one week look-ahead that they previously received," the White House said in its fact sheet. "This increased transparency will give state and local leaders greater certainty around supply so that they can plan their vaccination efforts and administer vaccines effectively and efficiently."

In his remarks, Biden repeated a phrase that the struggle to quell the pandemic is akin to "a wartime effort." He cautioned Americans that there are more months ahead of sacrifice, and he urged them to continue wearing masks and maintain social distancing. And he called on Congress to pass his $1.9 trillion rescue package to help struggling Americans and to provide billions in funding to aid in the vaccine distribution effort.

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