• The U.S. plans to buy 100 million additional doses of Johnson & Johnson's Covid-19 vaccine, two administration sources told NBC News.
• Biden is expected to make the announcement during a meeting at the White House with executives from J&J and Merck.

The U.S. plans to buy 100 million additional doses of Johnson & Johnson's Covid-19 vaccine, the Biden administration announced Wednesday.

President Joe Biden is expected to discuss the plans later Wednesday during a White House meeting with executives from J&J and Merck.

J&J currently has a deal with the U.S. government to provide 100 million doses by the end of June. The federal government shipped out nearly 3.9 million doses of the single-shot vaccine last week and says it plans to distribute 16 million more by the end of this month.

The new order allows "the president to plan for the future in the latter part of the year," Andy Slavitt, a senior advisor to Biden's Covid response team, said during a White House news briefing. "This is wartime," Slavitt said, adding more doses gives the administration "more flexibility for our upcoming needs."

In a statement, J&J noted that the government's initial deal of 100 million doses, worth $1 billion, in August gave it the option to purchase additional shots under a subsequent agreement.

"We look forward to any future discussions with the U.S. Government and to participating in the event at the White House later today," the company said in a statement.

The announcement comes as the administration works to ramp up production of J&J's vaccine after it learned earlier in the year that the company had fallen behind in output.

The Food and Drug Administration on Feb. 27 authorized J&J's vaccine for use in people 18 and older. Unlike Pfizer's and Moderna's vaccines, J&J's one-dose regimen eliminates the need for patients to return for a second dose, and it can be stored at refrigerator temperatures for months.

The New York Times first reported in January that unexpected manufacturing delays would lead to a reduced initial supply of J&J's drug if it were given emergency authorization.

Last week, Biden announced that pharmaceutical giant Merck would help make J&J's Covid vaccine. Under the arrangement, Merck will dedicate two facilities in the U.S. to J&J's vaccine. One will make the vaccine and the other will provide "fill-finish" services, when the vaccine is placed in vials.

The Department of Health and Human Services said the U.S. will use the Defense Production Act to give Merck $105 million to convert, upgrade and equip the company's facilities to the standards necessary to safely manufacture the vaccine.

White House Chief Medical Advisor Dr. Anthony Fauci said last month he was "disappointed" with the number of doses initially expected from J&J, adding the federal government thought there would be "considerably more."

"It may take until June, July and August to finally get everybody vaccinated," Fauci told CNN on Feb. 16. "So when you hear about how long it's going to take to get the overwhelming proportion of the population vaccinated, I don't think anybody disagrees that that's going to be well to the end of the summer and we get into the early fall."