Only ‘a few million’ Johnson & Johnson doses will be ready when the vaccine is likely to be authorized, the White House says.

Johnson & Johnson, whose Covid-19 vaccine is expected to be federally authorized for emergency use by the first week in March, will have only “a few million” doses on hand for distribution, a key White House adviser said Wednesday.

Though the vaccine would be the third to be authorized in the United States, the nation’s vaccine stock would not immediately increase. That is disappointing news for health officials and residents across the country who were hoping that another supplier would add to the 13.5 million doses a week now being delivered to states by Pfizer-BioNTech and Moderna, the two vaccines now federally authorized.

Even so, President Biden said on Tuesday evening that every American who wanted a Covid-19 vaccination would be able to get one by the end of July, offering a more encouraging forecast than he delivered a week earlier when he warned that logistical and distribution hurdles would delay vaccinations beyond the end of the summer. Mr. Biden, who made the comment in Milwaukee during a town-hall-style meeting hosted by CNN, then qualified the remark slightly, saying that the doses would “be available” by then. But he also said he did not expect it to take months to get the shots into people’s arms.

Johnson & Johnson’s vaccine has several advantages over Pfizer’s and Moderna’s. It is only one dose instead of two and can stay viable in a refrigerator for three months, while the other two have to be kept frozen.

But despite the federal government’s plan to compress the timetable for vaccine development by helping companies manufacture vaccines while they tested them in clinical tests, Johnson & Johnson is still struggling to ramp up production at its plant in Baltimore, Md.

Jeffrey D. Zients, the White House’s chief pandemic adviser, noted Johnson & Johnson began its work under the Trump administration. Under its contract, it is supposed to deliver 100 million doses by the end of June — with 37 million due by the end of March.

Its deliveries will be “more back end loaded,” Mr. Zients said. “We are doing everything we can to accelerate the delivery schedule.”