The White House publicly acknowledges the U.S. is likely to miss Biden’s July 4 vaccination goal.

The White House on Tuesday publicly acknowledged that President Biden does not expect to meet his goal of having 70 percent of adults at least partially vaccinated by July 4 and will reach that milestone only for those aged 27 and older.

It would be the first time that Mr. Biden has failed to meet a vaccination goal he has set. If the rate of adult vaccinations continues on the current seven-day average, the country will come in just shy of Mr. Biden’s target, with about 67 percent of adults partly vaccinated by July 4, according to a New York Times analysis.

White House officials have argued that falling short by a few percentage points is not significant, given all the progress the nation has made against Covid-19. “We have built an unparalleled, first-of-its-kind, nationwide vaccination program,” Jeffrey D. Zients, the White House pandemic response coordinator, said at a news briefing. “This is a remarkable achievement.”

In announcing the goal on May 4, Mr. Biden made a personal plea to the unvaccinated, saying getting a shot was a “life and death” choice. According to the latest figures from the Centers for Disease Control and Prevention, 150 million Americans have been fully vaccinated and 177 million have received at least one dose.

Young adults aged 18 to 26 have so far proven particularly hard to persuade. Younger Americans are less likely to be vaccinated than their elders, and factors like income and education may affect vaccine hesitancy, according to two new studies by the C.D.C.

“The reality is many younger Americans that felt like Covid-19 is not something that impacts them, and they’ve been less eager to get the shot,” Mr. Zients said.

He said it would take “a few extra weeks” to reach more of that group to achieve the goal of 70 percent of adults at least partially vaccinated.

Mr. Zients and Dr. Anthony S. Fauci, the nation’s top infectious disease expert, both stressed that the administration’s efforts would continue long after that benchmark is reached. Seventy percent “is not the goal line, nor is it the end game,” Dr. Fauci said. “The end game is to go well beyond that, beyond July 4 into the summer and beyond, with the ultimate goal of crushing the outbreak completely in the United States.”

But health experts warn that the falloff in the vaccination rate could mean renewed coronavirus outbreaks this winter when cold weather drives people indoors, with high daily death rates in areas where comparatively few people have protected themselves with shots.

“I give credit to the Biden administration for putting in place a mass vaccination program for adults that did not exist,” said Dr. Paul Offit, the director of the Vaccine Education Center at Children’s Hospital of Philadelphia. “But now we’ve hit a wall.”

In recent weeks, new infections, hospitalizations and deaths related to the virus have declined sharply nationwide. As of Monday, the seven-day average of new virus cases across the United States was 11,243 cases a day, a nearly 30 percent decrease over the last two weeks, according to a New York Times database.

But Dr. Fauci also warned of the rising prevalence of the Delta variant, first identified in India, which is more contagious than previous versions of the virus and may cause more severe disease. The variant now accounts for an estimated 20 percent of new infections, he said.

The vaccines authorized in the United States are effective against the Delta variant, and he said the increase lent urgency to the campaign to vaccinate as many Americans as soon as possible.

Unless tens of millions more Americans get vaccinated in the next few months, Dr. Offit said, “I think, come winter, we are going to again see a surge. And that surge is going to occur exactly where you would expect it to occur — in areas that are unvaccinated or under-vaccinated.”

Dr. Fauci said that “there is a danger, a real danger, that if there is a persistence of a recalcitrance to getting vaccinated that you could see localized surges” or regional spikes.

But he added: “I don’t think even under those circumstances that you’re going to see things like 1,000 deaths a day.” At the height of the post-holiday surge in January, the known daily death toll in the United States exceeded 4,000.

Lazaro Gamio contributed reporting.