President Biden said Thursday that his administration had finalized deals for another 200 million doses of the two coronavirus vaccines authorized in the United States, giving the country enough vaccine by the end of July to cover every American adult.

In remarks capping an afternoon tour of the National Institutes of Health, Biden said the federal government had purchased 100 million more doses from Pfizer and German company BioNTech, as well as 100 million more from Moderna, using options built into existing contracts with those companies.

The announcement was the centerpiece of an emotional address from Biden, who made a point of speaking through his mask as he called it a “patriotic responsibility” to wear one.

“We remain in the teeth of this pandemic,” he said, observing that January was the deadliest month of the pandemic, in which “we lost over 100,000 of our fellow citizens.” Mutations of the virus pose new challenges, he said, even as infections and hospitalizations begin to decline.

Recalling a conversation with a nurse staffing an Arizona vaccination site, Biden said she described inoculating people against the coronavirus as “like administering a dose of hope.”

“We’re going to get those doses of hope out,” Biden promised.

The new deals don’t immediately expand access to shots, which remain in short supply throughout much of the country. They primarily serve to prevent a shortfall later in the year by increasing supply by 50 percent, bringing the total to 600 million doses.

President Biden on Feb. 11 announced that the U.S. will have enough vaccine doses available for 300 million Americans by the end of July. (The Washington Post)

Because both products are two-dose regimens, that will be enough to fully vaccinate 300 million people. An estimated 260 million people in the United States are currently considered eligible to receive a coronavirus vaccine, though trials involving children as young as 12 could widen the pool.

Together, Pfizer and Moderna had already agreed to provide 400 million doses to the United States. Biden said some of those doses would be delivered sooner than anticipated, guaranteeing enough supply by the end of May to vaccinate 200 million people under the two-dose protocols. Pfizer had already expedited its delivery schedule for those doses, and a Moderna spokesman confirmed Thursday that the Cambridge-based company could do the same.

Moderna issued a statement confirming the purchase and saying it was “working with its domestic manufacturing partners,” as well as federal regulators, to “explore ways to accelerate delivery, with the goal of providing this new order of 100 million doses before the end of July 2021.” Pfizer spokeswoman Amy Rose confirmed the purchase as well as the timeline Biden outlined.

In securing the additional doses, the government used options built into contracts negotiated last year by the Trump administration. Biden said last month that he would seek the additional doses, part of a strategy to double down on the two vaccines that have already won federal clearance and not count on candidates from other companies becoming available.

With his vow that the additional doses would be available by the end of July, the president noted that the delivery would be “faster than we expected.” His advisers had previously indicated that a summer delivery was anticipated, and Trump administration officials said they were operating under the assumption of a quarterly schedule, with an additional tranche possible by August or September.

Biden administration officials have been telling partners that their recent move to expand Pfizer’s priority rating under the Defense Production Act would help the pharmaceutical giant obtain needed equipment to produce the additional doses sooner than anticipated, according to people with knowledge of the discussions.

Asked last week about the action under the Korean War-era law, Rose did not address it directly but said, “Our teams continue to work closely on our production as our commercial ramp-up progresses.” A Pfizer executive said in a recent interview with USA Today that the company expects to halve its production time as it gains familiarity with the process, expediting the availability of its product globally.

As the country seeks to stay ahead of the spread of more-transmissible and possibly more-lethal variants, top health officials have expressed confidence that widespread inoculation would soon be possible because of a steady ramp-up in manufacturing.

“By the time we get to April, that will be what I would call . . . ‘open season,’ namely, virtually everybody and anybody in any category could start to get vaccinated,” Anthony S. Fauci, the nation’s leading infectious-disease expert, said Thursday on NBC’s “Today” show.

The Biden administration has already increased weekly state allocations by nearly 30 percent, though shortages remain pronounced in many areas. Los Angeles Mayor Eric Garcetti (D) said Wednesday that the city would temporarily close a mass vaccination site at Dodger Stadium, along with several other locations, because there weren’t enough doses.

Additional doses are also expected to come from Johnson & Johnson, which submitted its application for a single-shot coronavirus vaccine to U.S. regulators earlier this month. If approved, the easy-to-store vaccine would further augment supply, although production setbacks are expected to limit availability until the spring. It’s also possible that vaccine experts may only recommend the vaccine for people in certain age groups.

Johnson & Johnson’s vaccine showed strong protection against severe disease from the variant first discovered in South Africa, but offered less-robust protection against moderate illness. Evidence from laboratory tests suggests the Pfizer and Moderna vaccines work against variants, but the ability of the immune response to block the South African variant is diminished.

As more vaccine becomes available, new challenges, including staffing, will arise. But health officials say they’re preparing themselves.

“We’re strategizing around that now and the good thing is we’ve had a couple of months of experience doing this that we can learn from,” said Kevin Litten, a spokesman for the Louisiana Department of Health.

Carolyn Y. Johnson and Amy Goldstein contributed to this report.