Johnson & Johnson is also in preliminary talks with Merck, a large American pharmaceutical company, about the use of its production lines, one of several ideas that federal health officials have discussed with the Biden transition team. Federal officials are keen to bolster the country’s vaccine manufacturing in the long term, and Merck’s facilities may be among the few with manufacturing capacity left.

Dr. McClellan, who sits on the Johnson & Johnson board of directors, said it would take months to adapt Merck’s factory to manufacture Johnson & Johnson’s vaccine. A senior administration official predicted this could take until the end of the year.

Other vaccine makers could also get through by mid-year. Novavax has worked to iron out the recently serious manufacturing issues that delayed its clinical trials. Moncef Slaoui, the chief scientific advisor to the Trump administration’s vaccine development program, said in a recent interview that Novavax could apply for emergency approval in late April. The government has already ordered 110 million doses of the Novavax vaccine to be dispensed by the end of June, and Novavax believes it can achieve that goal.

Mr Biden has surrounded himself with new health officials charged with bringing vaccines from factories to recipients, including Dr. Bechara Choucair, the former Chicago health commissioner who is the White House’s vaccination coordinator; and Tim Manning, a former top Federal Emergency Management Agency officer who is now the supply coordinator. Dr. David A. Kessler, former FDA commissioner, will lead the federal government’s vaccine development program at the Department of Health and Human Services, with a special focus on manufacturing.

After Pfizer-BioNTech and Moderna vaccines proved highly effective in clinical trials late last year, the Trump administration considered reconsidering its strategy of supporting six different vaccine manufacturers and instead throwing all its weight behind the tried and tested manufacturers. One senior administrator described “countless hours of debate” on the subject.

In the end, officials decided that it was important to keep targeting a broad vaccine portfolio, also because no one had figured out which vaccines would be best for children or most effective against emerging variants. They recommended that the Biden administration do the same.

Katie Thomas and Donald G. McNeil Jr. contributed to the coverage.