The announcement comes as the White House looks to speed the production of the single-dose vaccine. Officials have said J&J faced unexpected production issues with its vaccine and produced only 3.9 million doses ahead of its receiving emergency use authorization on Saturday. The company says it is on pace to deliver 100 million doses by the end of June.

Facing questions about the company’s slipping delivery schedule, J&J Vice President Richard Nettles told lawmakers on Capitol Hill last week that the company had faced “significant challenges” because of its “highly complex” manufacturing process.

The assistance from Merck was expected to help J&J meet its production commitments and expand supply even further, but the administration did not immediately provide specifics.

President Joe Biden is set to highlight the development in a speech Tuesday afternoon, as his administration now expects to have enough supply of the three approved vaccines to inoculate all eligible American adults by June — though actually delivering the injections could take longer.

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The official confirmed Merck’s involvement on condition of anonymity ahead of Biden’s public announcement. The news was first reported by The Washington Post.

It was not immediately clear when the effect of Merck’s assistance would be reflected in supply. Previously, federal officials have cautioned that setting up the highly specialized manufacturing lines to produce vaccines would take months.

Merck halted its own plans to develop a coronavirus vaccine earlier this year, finding that their candidates were generating an inferior immune system response compared with other vaccines. It said it would instead focus its work on developing treatments for COVID-19.

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Compared to the two-dose versions produced by Moderna and Pfizer, the J&J vaccine is less resource intensive to distribute and administer, making it a critical piece to U.S. plans to spread vaccinations around the world — but only once Americans are inoculated. The J&J vaccine can be stored for months at refrigerated temperatures, rather than frozen, and doesn’t require patients to return for a second dose three or four weeks later.