The US will purchase 200 million additional COVID-19 vaccine doses and pledges to get states 10 million shots per week

• The Biden administration announced Tuesday that it plans to purchase another 100 million COVID-19 vaccine doses each from Pfizer and Moderna.
• That would bring the total US vaccine supply to 600 million doses — enough to vaccinate 300 million Americans.
• The administration also pledged to distribute at least 10 million weekly doses to states, tribes, and territories for the next three weeks.

President Joe Biden has been ramping up his coronavirus vaccination ambitions.

Biden kicked off his administration with a promise to get 100 million coronavirus shots into arms within 100 days. That would require at least 1 million vaccinations per day. On Monday, he raised his goal to 1.5 million shots a day.

Then, on Tuesday, Biden announced that he had directed his COVID-19 Response Coordinator, Jeffrey Zients, to purchase another 100 million vaccine doses each from Pfizer and Moderna. This would add to the 400 million total doses previously purchased from the two companies.

The new supply, which is expected to be delivered this summer, means the US could have enough doses to fully vaccinate 300 million Americans with the two-dose vaccines by the end of the summer.

The US was administering 983,000 daily shots, on average, as of Friday. Several states have reported vaccine shortages in recent weeks, which forced some to cancel vaccine appointments.

Read more: What's coming next for COVID-19 vaccines? Here's the latest on 11 leading programs.

Starting Tuesday, the Biden administration also promised to distribute at least 10 million doses per week to states, tribes, and territories - up from the current rate of 8.6 million doses per week. The administration said it would guarantee these doses for the next three weeks. Moving forward, officials added, states will know how many doses to expect three weeks ahead of time.

The ability to ramp up this supply comes primarily from an increase in available Moderna shots, a senior Biden administration official said on a Tuesday press call.

Pfizer's vaccine distribution is also ahead of schedule: The company's CEO, Albert Bourla, told Bloomberg on Tuesday that Pfizer was on track to deliver 120 million doses to the US in the first quarter of 2021 - 20 million more doses than it initially planned. That's because Pfizer's vaccine label now instructs healthcare workers to extract six doses from each vial, as opposed to the original five. French pharma giant Sanofi also recently struck a deal to help Pfizer produce more than 100 million doses.

Still, vaccines will only be available to priority groups for several more months.

"We wish we could say today that every American who wanted to be vaccinated could go get vaccinated. That's clearly not the case," the administration official said on the Tuesday call. "That's not the level of supply that we found when we arrived and it's going to take a number of months for us to be in a position where we can actually say to Americans that it is 'open season,' as Dr. Fauci calls it."

Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, recently told the Harvard Business Review that the US could potentially start giving shots to anybody who wants one in April.

"By the time we get to the beginning of the fall, we should have that degree of protection that I think can get us back to some form of normality," Fauci said.