The pact should quickly boost the supply of Johnson & Johnson’s single-shot vaccine, said Biden officials, who spoke to the Washington Post under the condition of anonymity. 

This comes after news that the rollout of Johnson & Johnson vaccine—the third to be federally approved for emergency use in the United States—would be slower than initially anticipated.

Merck will dedicate two U.S. facilities to manufacturing the Johnson & Johnson shot and will be aided by funding from the Defense Production Act, according to the White House. One of the Merck facilities will make the vaccine, which Biden officials said could potentially double the supply of what Johnson & Johnson could make on its own. The other will provide “fill-finish” services, which is the final manufacturing process in which the vaccine is placed into vials and packaged for distribution.

“Our objective is to build on the incredible announcement that we now have three vaccines the American people can now have access to,” press secretary Jen Psaki said Tuesday ahead of Biden’s remarks.

The partnership was forged after it was apparent that Johnson & Johnson was far behind on its manufacturing goals. 

As early as this morning, approximately 3.9 million doses of the Johnson & Johnson vaccine were expected to arrive at health care providers across the country. After this initial 3.9 million—which represents the entirety of Johnson & Johnson’s supply—Biden officials warned over the weekend that it may be some time before there were more Johnson & Johnson vaccines ready for distribution.

Last year, the U.S. government signed a $1 billion contract with the company, which in turn pledged to deliver at least 100 million doses total and 37 million doses by the end of March.

The partnership between Merck and Johnson & Johnson will boost the production of the single-shot vaccine, but the timeline for when that help will come remains unclear. According to reporting from the Washington Post, it could take up to two months for the “fill-finish” plant to be ready for operation, and a few more to equip the other facility, which will make the vaccine.

In an interview with CNN on Monday, Johnson & Johnson CEO and chairman Alex Gorsky said he was confident that the company would be able to deliver on its pledge to meet the distribution goals for its vaccine.

“We’ve developed an extensive partnership here in the United States and Europe and other places around the world, and we’re very confident in our ability to deliver 20 million doses by the end of March and 100 million doses in the first half of the year en route to a billion doses by the end of this year,” Gorsky said. 

Johnson & Johnson’s vaccine comes at a crucial time. As Fortune reported on Monday, experts say getting as many people vaccinated as possible over the next few weeks will be crucial to tackling the pandemic’s biggest risk on the horizon: a proliferating array of COVID-19 variants (some of them more dangerous than the currently dominant strain), which could fuel a fourth pandemic surge.

With the help of Merck’s manufacturing plants, Johnson & Johnson’s efforts to distribute its vaccine received a necessary boost. The pharmaceutical company has a century-long history of vaccine production. 

In the United States, Merck—ranked No. 69 on the Fortune 500 list—is the sole supplier of a childhood combination vaccine that protects against measles, mumps, and rubella. Merck also produces Gardasil, and in 2019, the company won FDA approval for a vaccine against Ebola.