WASHINGTON — For much of the spring, President Biden touted his goal of vaccinating 70 percent of eligible adults by July 4. The United States missed that mark by about a month. Since then, he and his top aides have studiously declined to say what share of the American population they’d ultimately like to see vaccinated, even as they intensify pressure on the unvaccinated to get their shots.

“We’re going to try to get as many people in the country vaccinated as humanly possible,” White House press secretary Jen Psaki said last Friday, declining to delve into specifics. “We’re not going to put an end limit on that.”

At the present, 56 percent of Americans are fully vaccinated, according to the Centers for Disease Control and Prevention (that number includes all Americans, not just adults; the share of adults fully vaccinated is 65 percent). Although that doesn’t account for the millions who have natural immunity from having been sickened with COVID-19, the figure is far too low to prevent community transmission.

“We need to get to >75% of the total population fully vaccinated to achieve containment, based on the dozen countries that have already achieved that level,” Dr. Eric Topol, founder and director of the Scripps Research Translational Institute, wrote in an email to Yahoo News. And that, he added, was “assuming we don’t see a worse than Delta strain capable of more immune evasion of our vaccines and natural immune response to infections.”

To reach Topol’s goal — which many other epidemiologists share, though some believe the rate should be even higher than that — the United States would have to vaccinate 63 million more people in the coming months, a challenging proposition considering that vaccination rates are plummeting. Only about 230,000 people are getting shots daily, the lowest number since January, when doses were still scarce.

Although doses are now plentiful, the desire for them has waned. People have begun to seek out booster shots, but the nation remains mired in what Biden refers to as “the pandemic of the unvaccinated.” Some have bristled at that phrase, but the reality is that vaccinated people are much less likely to contract or spread the coronavirus.

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How long the pandemic continues depends almost entirely on what share of the population remains unvaccinated, not on the distribution of booster shots.

Moreover, the notion that boosters are needed in the first place could give unvaccinated Americans the impression that the vaccines are not effective. “There are so many people who are vaccine-hesitant or vaccine-resistant, and the booster rollout risks further eroding people’s trust in the vaccines,” says Dr. Lucy McBride, a Washington, D.C., physician who has fielded many booster-related questions from her patients.

“Boosters are important, but the most important thing we need to do is get more people vaccinated,” Biden said before receiving his own booster shot earlier this week.

To make matters even more complicated, children as young as 5 will soon become eligible for vaccinations, which will present the Biden administration with another front in the vaccine wars. Given the obvious sensitivities around children’s health, that front could be the most fraught of all, even though clinical trials have uniformly shown the vaccines to be safe for even the youngest recipients.

Talking about vaccination targets has become increasingly challenging. Throughout the spring, the U.S. emerged as a worldwide immunization leader, a fact that Biden routinely celebrated in remarks from the White House.

Since then, the European Union has solved the challenges that plagued its continent-wide rollout for much of the winter and early spring. The result is that Portugal is now the most vaccinated country in the world, while the United States is in 40th place.

“US is falling further and further behind,” recently tweeted the epidemiologist and Brown University public health dean Dr. Ashish Jha. “We’ve got to get our act together folks.”

To complicate matters even more, the vaccination gap in the United States is highly partisan. The racial gap that emerged early in the vaccination drive has effectively been closed, while the one between conservatives and progressives has grown.

“The partisan gap in COVID-19 vaccination rates has continued to widen, driving a higher death rate in Republican-leaning counties,” a recent internal Biden administration document obtained by Yahoo News warned, pointing to a Pew poll that found a 26 percent gap between the share of Democrats and Republicans who have received at least one dose of the vaccine.

The president’s own advisers know he is unlikely to persuade conservatives. Making concessions to that reality effectively cedes ground to Republicans like Gov. Ron DeSantis of Florida, whose new surgeon general has said the coronavirus vaccines are “nothing special.”

Asked about the divide during a recent White House pandemic-related briefing, adviser Dr. Marcella Nunez-Smith acknowledged that “the messenger matters.” But with many conservatives opposed to Biden’s vaccination mandates, it is not clear how many of them are willing to do that work on his behalf.

The conversation about vaccination targets is really a proxy for much larger questions that some public health officials have begun to ask: How much coronavirus are we willing to live with? How many months of high-alert living can Americans put up with? When do we declare the pandemic over, lifting pandemic-related public health restrictions once and for all?

“I don’t think we can eradicate COVID-19 by its nature, since it is highly transmissible,” says Dr. Monica Gandhi, an infectious disease specialist at the University of California, San Francisco. Gandhi told Yahoo News she believes that 67 percent is a reasonable vaccination target, pointing out that that was the benchmark for Norway’s lifting of all restrictions.

A person familiar with the White House insisted, as Psaki did, that focusing on numbers was misguided. “Our focus is less on a metric, more on the things that achieve the actual goal of saving lives,” that source said.

But the president is clearly focused on metrics. He talked about them last Friday, before leaving for Camp David, then again on Monday, as he rolled up his sleeve to get his third Pfizer shot.

“They’re the ones who’ve been saying a number all along,” notes Vinay Prasad, an oncologist at the University of California, San Francisco, who has emerged as a prominent critic of conventional thinking on the pandemic.

Like his colleague Gandhi, another iconoclast, Prasad is one of a growing number of medical experts who believe we need to be thinking about the next stage of the pandemic, one in which we learn to coexist with the coronavirus.

“I don’t think we should use vaccination percentages as an endpoint for lifting restrictions, in part because it discounts natural immunity,” says McBride, the Washington physician. “We should use regional hospitalization or death rates, and not percent vaccination, to define the endpoint of the pandemic.”

Hospitalizations and death rates have both been falling, suggesting that the Delta wave has crested, at least across the South and lower Midwest. Whether the cold weather brings another wave will become apparent soon enough.

To be sure, the United States is heading into the final months of 2021 much better positioned than it was a year ago. But the millions of unvaccinated Americans represent a kind of collective question mark, one that leaves open the possibility of the pandemic continuing into 2022.

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