President Biden said Wednesday his administration is ordering an additional 100 million doses of the single-shot vaccine developed by Johnson & Johnson, another step to amass stockpiles as a safeguard against manufacturing disruptions or outbreaks of coronavirus variants.

Biden announced the plan a week after vowing that the United States will have enough vaccine to administer to all American adults by the end of May.

The new order, which Johnson & Johnson indicated has not been finalized, will bring total U.S. orders of the Johnson & Johnson vaccine to 200 million doses. Because it’s a one-dose vaccine, the supply will represent enough for 200 million people.

It is not expected to be fulfilled until the second half of the year, administration officials said.

The U.S. government has ordered 600 million doses of two-dose vaccines from Pfizer-BioNTech and Moderna, allowing for 300 million people to be fully vaccinated by the end of July, or a total of 500 million people with all vaccines ordered to date. The number of adults estimated to eligible to be vaccinated in the United States is 260 million.

Biden, responding to growing criticism about the widening gap with developing nations as the United States and Europe accelerate immunization efforts, said his goal is to take care of the pandemic in the United States, then share vaccine with other countries.

“We’re going to start off making sure Americans are taken care of first, and we’re then going to try to take care of the rest of the world,” the president said in response to a shouted question from a reporter.

He said he would share any U.S. surplus with the rest of the world. But in his remarks, he made clear he did not consider the doubled Johnson & Johnson supply to be surplus.

“We need maximum flexibility. There is always a chance we will encounter unexpected challenges or there will be a new need for a vaccination effort,” he said.

The head of the World Health Organization, Tedros Adhanom Ghebreyesus, warned in January that the world faces a “catastrophic moral failure” with the global vaccine gap between rich and poor nations. He said it was unfair for younger, healthy people in richer countries to be immunized before the elderly and infirm in nations with fewer resources.

Epidemiologists also warn that variants will emerge in unvaccinated populations around the world and will probably arrive on U.S. shores if allowed to rage unchecked.

Biden acknowledged that scientific concern Wednesday. He also noted that the United States has agreed to give $4 billion to Covax, a nonprofit international funding mechanism to purchase vaccines for developing nations.

But senior administration officials have stressed the importance of redundancy in domestic vaccine supply, given the uncertainty involved in manufacturing new products at scale, as well as the possible need to give people booster shots to guard against variants of the virus, and the potential authorization of shots for children under 16 early next year.

“This order allows the president to plan for the future in the latter part of the year,'' senior White House adviser Andy Slavitt said Wednesday at a briefing about the government response to the coronavirus. “This is wartime, and as facts still emerge it gives us maximum flexibility for our upcoming needs.”

Biden previewed the plans during an afternoon meeting with executives from Johnson & Johnson and Merck, just over a week after announcing his administration had helped broker a deal between the rival companies to speed up production of Johnson & Johnson’s vaccine.

Johnson & Johnson has agreed to supply the United States with its first 100 million doses by the end of June, and its latest goal is to produce 94 million of those doses by the end of May.

Merck said it will receive $268 million from the federal government to pay for required upgrades to multiple facilities to handle the work of creating raw vaccine substance as well as fill-and-finish operations to place it in vials and prepare it for shipping.

Getting Merck geared up for the job was expected to take several months, with the fill-and-finish operations beginning first. Johnson & Johnson’s vaccine is also being made at Emergent BioSolutions, a contract manufacturer in Baltimore. Emergent said it has the capacity to make enough raw vaccine substance for up to 1 billion doses of Johnson & Johnson vaccine in 2021.

Johnson & Johnson’s vaccine is priced at $10 a dose. The company has said it does not intend to make any profit. Its original deal for 100 million doses for the United States struck last August contained a provision allowing the government to buy up to 200 million more doses, options that would be subject to further negotiations in an additional stand-alone agreement, according to the contract.

The company indicated Wednesday that the new order was not a done deal.

“The U.S. government has the option to purchase additional doses under a subsequent agreement. We look forward to any future discussions with the U.S. government,” Johnson & Johnson said in an emailed statement Wednesday morning.