The shining light at the end of the pandemic tunnel has long been the point at which large enough portions of the population are immunized against the coronavirus that the virus itself can no longer spread widely, because there aren’t enough hosts who can be infected left to keep transmission rates up. “Herd immunity” wouldn’t necessarily eliminate the virus entirely, but public health officials say it would drastically cut down on transmission and make it easier to contain.

Early in the pandemic, officials said there were two ways of achieving herd immunity. One was to let the virus intentionally pass through the population until enough people had recovered, and developed protective antibodies, that transmission went down — a strategy doctors called “morally reprehensible” because it would cost unnecessary lives. But it was a strategy a prominent adviser to President Donald Trump pushed for heavily — though unsuccessfully. Sweden actually tried it — and ended up with some of the highest infection and death rates in the world, without the economic relief lawmakers wanted.

The other way to achieve herd immunity is to vaccinate a large portion of the population. Exactly what portion of the population is open to question; Francis Collins, director of the National Institutes of Health, told The Washington Post in January it would require immunizing “80, 85 percent” of the country. Other estimates put it as low as 70 percent. But the United States isn’t close to either number right now: About 44.4 percent of the population had received at least one vaccine dose as of Monday afternoon.

As the country rapidly approaches a period in which everyone who wants a vaccine has gotten one, the big challenge to achieving herd immunity becomes persuading people who are skeptical or unwilling to get vaccinated to get their shots. And public health officials have been sounding the alarm for weeks that the United States might not ever actually meet that original goal.

“I don’t know that we ever achieve true herd immunity, where this virus just stops circulating,” former Food and Drug Administration commissioner Scott Gottlieb said on CNBC last month. “I think it’s always going to circulate at a low level.”

It’s not that the opportunity isn’t there. The United States has far more vaccine doses available than are currently in demand and has contracts with drugmakers for hundreds of millions more doses than needed to vaccinate everyone in the country. The problem is that the number of unvaccinated Americans willing to get their shots is dwindling.

According to a recent Washington Post-ABC News poll, a majority of U.S. adults who hadn’t been vaccinated said they either probably or definitely wouldn’t get inoculated. That group represents almost a quarter of U.S. adults.

It was always hard to define specific metrics for herd immunity, in part because the portion of the population that needs to be immunized varies depending on the transmissibility of the disease. But officials seem to want to get rid of the term altogether and instead simply focus on vaccine distribution and outreach to hesitant American adults.

“I would like to get people away from this concept of referring to something that it is very elusive in its definition,” said Anthony S. Fauci, the country’s top infectious-disease expert, “We’ve made estimates that [herd immunity] is somewhere between 70 and 85 percent, but we don’t know that as a fact. So that rather than concentrating on an elusive number, let’s get as many people vaccinated as quickly as we possibly can.”

That might be easier said than done. The split of vaccinations is increasingly partisan, and vaccine-hesitant Trump voters who participated in a recent focus group said their skepticism is worsening.

That leaves Fauci, Biden and other officials — including Republicans — to sway those skeptics. But as time goes on, as the economy begins to reopen, that might get harder and harder. If true herd immunity isn’t achievable, the goal becomes reducing cases as much as possible — but even that will be hard if tens of millions of U.S. adults refuse to get vaccinated.