One of the few missteps by HHS on the vaccine front has effectively been reversed. The Trump administration has reached an agreement with Pfizer and its partner BioNTech to supply an additional 100 million doses by the end of July, with most of those delivered by the end of the second quarter. The move comes after criticism of an earlier decision not to exercise a purchase option that would have secured the doses up front:

The Trump administration has reached an agreement with Pfizer and BioNTech to purchase 100 million additional doses of its coronavirus vaccine, which will be fully delivered no later July 31, 2021, the Department of Health and Human Services announced Wednesday.

Why it matters: The deal will allow millions more Americans to be vaccinated at zero cost by next fall. Under the terms of the agreement, 70 million of the 100 million doses purchased will be delivered by June 30, 2021.

That now puts the commitment from Pfizer to the US at 200 million doses, enough to inoculate about a third of the country in the two-dose regimen. This time, HHS also negotiated options for another 400 million doses, which if exercised would cover most of the rest of the population.

How likely are we to need those options? As Axios notes, HHS has already purchased hundreds of millions of doses from other manufacturers through Operation Warp Speed. Moderna alone has 200 million committed by the end of June. AstraZeneca, which is finishing its Phase 3 trial soon, has 100 million set aside for the US, as does Johnson & Johnson, Novavax, and Sanofi/GlaxoSmithKline. Obviously, the US bought those doses up front as a backup in case one or more of the vaccine candidates failed. If they all succeed, we won’t need the Pfizer options at all.

That might have been why HHS declined the earlier options, too:

The administration’s new agreement comes as hundreds of millions of people in the U.S. wait to be vaccinated. Earlier this month, Dr. Scott Gottlieb, a former FDA commissioner and a member of Pfizer’s board of directors, said that the White House declined “multiple” offers from the company to strike a deal on more vaccine doses for the second quarter of 2021.

Better to be safe than sorry, especially since Pfizer and Moderna represent the bird in the hand rather than two in the bush. If we end up with extra, we can redirect some of our supply to other countries with resource gaps. At the moment, though, this deal guarantees that the US will have enough supply to get most of the country vaccinated by the end of the second quarter at the latest. If the other manufacturers win approval soon — and both AstraZeneca and Johnson & Johnson are said to be getting close to the end of their Phase 3 trials — it might be much sooner than that.