The announcement comes as the White House looks to speed the production of the single-dose J&J vaccine and accelerate the nation's plans to reach "herd immunity" in the US and begin restoring normalcy after the pandemic. Biden noted that vaccine supply was only one bottleneck toward that goal, and that the new challenge will be injecting doses into arms as swiftly as possible. 

To that end, the Biden administration told governors Tuesday to prepare for their supplies of vaccine to continue to climb over the coming weeks. Additional doses are also heading toward a federally backed program to administer doses in more accessible retail pharmacies. 

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 has promised to deliver 100 million doses by the end of June. 

White House press secretary Jen Psaki also announced Tuesday that the federal government was increasing supply of the Moderna and Pfizer vaccines to states next week to 15.2 million doses per week, up from 14.5 million previously. States will also receive 2.8 million doses of the J&J shot this week. 

On a call with governors Tuesday, White House coronavirus coordinator Jeff Zients said states should prepare for administering 16-17 million total weekly doses of Pfizer and Moderna vaccines by the end of March, climbing to 17-18 million weekly by early April. The supply of J&J doses to states, expected to dip after the initial shipment this week, will climb to 4-6 million weekly doses by the end of March and 5-6 million doses weekly through the end of April. 

More than 800,000 doses of the J&J vaccine will also be distributed this week to pharmacies to administer in a separate federally-run program that also includes 2.4 million doses of the other two shots. Both figures are expected to steadily increase, as the White House increasingly looks to the capacity of pharmacy chains like CVS and Walgreens to help speed the nation's mass vaccination campaign. 

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. 

Psaki said that an "across the administration effort" was required to get the two historic rivals to work together on the vaccines, even though conversations between the two companies have been going on for months. 

"There's a difference between conversations and it moving forward," she said. 

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. 

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. 

The White House said Merck would devote two plants to the production process. One would make the vaccine and the other would handle inserting the vaccine into vials and ensuring strict quality controls. Psaki said the Biden administration was using its powers under the Defense Production Act to help Merck retool to work on the production. 

The news was first reported by The Washington Post. 

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 US 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. 

J&J has set up a global production network that includes brewing bulk vaccine at its Janssen facility in the Netherlands, and with a company in the US, Emergent BioSolutions, and another in India, Biological E. Ltd. Other contract manufacturers are lined up to help with later steps, including putting the vaccine into vials, in the US, Italy, Spain and South Africa. 

In the scramble to create COVID-19 vaccines, the three Western drug makers who've dominated the vaccine industry for decades — Merck & Co., Sanofi and GlaxoSmithKline — surprisingly all fell short. 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.