WASHINGTON — The Biden administration now expects to begin a Covid-19 booster campaign with retooled vaccines in September because Pfizer and Moderna have promised that they can deliver doses by then, according to people familiar with the deliberations.

With updated formulations apparently close at hand, federal officials have decided against expanding eligibility for second boosters of the existing vaccines this summer. The new versions are expected to perform better against the now-dominant Omicron subvariant BA.5, although the data available so far is still preliminary.

So far, only Americans over 50 and those over 12 with certain immune deficiencies have been eligible for second booster doses. Although some federal officials pressed to bolster the protection of younger Americans now, officials agreed on the goal of strengthening everyone’s immunity in the fall with what is hoped to be a more effective booster, ahead of a possible winter surge of the virus.

In internal deliberations, some senior health officials argued that eligibility for a second booster should be broadened before the reformulated version is ready because coronavirus infections are on the rise again. Dr. Anthony S. Fauci, the president’s chief medical adviser, and Dr. Ashish K. Jha, the White House pandemic response coordinator, both advocated that position.

“I think there should be flexibility and permissiveness in at least allowing” a second booster for younger Americans, Dr. Fauci said in an interview this month. One alternative discussed was offering the shots only to a subset of younger, at-risk individuals, such as pregnant women.

But officials at the Food and Drug Administration and the Centers for Disease Control and Prevention argued that the government should concentrate instead on the fall campaign with updated doses — if the campaign could begin soon enough. After both Pfizer and Moderna assured the F.D.A. recently that they could deliver millions of doses by mid-September, regulators decided it was better to wait for those shots.

All adults are expected to be eligible for the updated booster shots. Children could be eligible as well, according to people familiar with the deliberations.

The federal government also plans to continue to stress that anyone who is eligible for additional shots should get them now, and not wait for the fall. As of midweek, health officials were still working out their specific advice about the reformulated shots.

One concern was assuring that people did not get a booster now and another, with the reformulated version, too soon after. Officials worried that, especially for young men, two boosters in close succession might elevate the risk of a rare heart-related side effect, myocarditis, that has been linked to both Pfizer’s and Moderna’s vaccines.

For other reasons, immunologists warn against receiving booster shots in short intervals.

“You can’t get a vaccine shot Aug. 1 and get another vaccine shot Sept. 15 and expect the second shot to do anything,” said Shane Crotty, a virologist at the La Jolla Institute for Immunology. “You’ve got so much antibody around, if you get another dose, it won’t do anything.”

“The antibodies stop that next dose from working” if the next dose is given too early, he added — a pattern that applies to other vaccines, such as those in tetanus or flu shots, as well.

Federal officials were also concerned about the public’s patience with additional shots. The number of recipients has been dropping with each new one offered. While nearly half of those eligible for the first booster opted to get it, for example, fewer than 30 percent of eligible Americans have chosen to receive the second booster — their fourth shot in total.

The Biden administration has been busy contracting for the newly designed doses. The Department of Health and Human Services recently made an advance purchase of 105 million doses of Pfizer-BioNTech’s vaccine for $3.2 billion, timed for possible deployment in the fall. The administration is expected to finalize a similar arrangement with Moderna soon.

The government’s decision comes as cases of the highly contagious BA.5 variant remain high across the country. Deaths and hospitalizations have risen in recent weeks. The number of new cases announced each day has hovered near 130,000 — likely a significant undercount because of the number of home tests that go unreported — and President Biden just had his own bout with the variant.

Deaths from Covid-19 are still heavily concentrated among older age groups, while hospitalizations remain well below the peak of the Omicron wave last winter.

At a late June meeting of an F.D.A. advisory committee, independent vaccine experts overwhelmingly agreed with the need to update the coronavirus vaccines because the virus is now more deft at dodging their protection. But both Pfizer and Moderna were reluctant to commit to delivering doses with a revised formulation at the start of fall.

Kathrin Jansen, the head of vaccine research for Pfizer, said at the meeting that her company was prepared to deliver doses by early October. Dr. Stephen Hoge, the president of Moderna, said his company would be able to deliver reformulated shots only by late October or early November.

But more recently, both companies assured federal officials that they could speed up their timetables and be ready in early September, according to people familiar with the discussions.

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