Public officials have long suggested the Covid-19 epidemic will end when America reaches so-called "herd immunity"—but now, with a sizable fraction of U.S. residents saying they won't get vaccinated, some experts now worry herd immunity is impossible.

Good? Bad? Ugly? We've updated our take on what's next for the epidemic.

Why herd immunity might not happen

There's still no consensus about the level of population-wide immunity to Covid-19 required to reach herd immunity, a term that describes the fraction of a population that must be immune to a disease to prevent outbreaks from growing rapidly.

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The World Health Organization last summer said that herd immunity likely would be reached when 60% to 70% of the population had been infected or vaccinated. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases and chief medical advisor to the White House, has suggested the threshold in the United States is around 75% to 85%.

But Fauci and other experts are now shying away from citing herd immunity as a goal. According to USA Today, that's because a significant fraction of Americans now say they won't get vaccinated.

For instance, a Kaiser Family Foundation (KFF) poll found that, as of March, 13% of U.S. adults said they will definitely not get vaccinated; 7% said they would get vaccinated only if required by their workplace, school, or other activities; and a further 17% want to "wait a while and see how it's working for others" before getting vaccinated.

Vaccine hesitancy is especially high in rural areas. KFF found that rural Americans were 11 percentage points more likely to say they definitely will not receive a Covid-19 vaccine than city-dwelling Americans.

In response to limited demand, officials in Mercer County, Ohio and Albany, Georgia, have gotten rid of mass vaccination sites. In other areas, such as Billings, Montana—the state's largest city—about 75% of vaccine appointments are open, according to Sen. Jon Tester (D-Mont.).

"There are states where they feel they have hit the wall," Mike Fraser, executive director of the Association of State and Territorial Health Officials, said. "The folks [who] wanted it have found it. The folks [who] don't want it are not bothering to find it."

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The hesitancy appears to be increasingly along political lines, the Washington Post reports, with more than 40% of self-identified Republicans saying they don't think they will get vaccinated.

"There are going to be places, rural Idaho, for example, where you have very independent-thinking people where there may be continuing spread, because you only get up to 25% of people vaccinated," William Schaffner, a professor and infectious disease expert at the Vanderbilt University School of Medicine, said. Citing the "striking" divide between urban and rural areas, Schaffner added that he's "really concerned this virus is going to continue to smolder in rural areas."

According to KFF's analysis, the United States will likely meet the demand of all adults seeking a first dose of a Covid-19 vaccine in about two to four weeks. "It appears we are quite close to the tipping point where demand for rather than supply of vaccines is our primary challenge," the authors of the report wrote.

If herd immunity is not reached, USA Today reports, the likely consequence is that Covid-19 outbreaks will continue indefinitely, and that people who are unvaccinated or have suppressed immune systems will remain at particular risk for serious complications.

How the US is working to combat vaccine hesitancy

Some public health and infectious disease experts say that, even if herd immunity is out of reach, the United States should focus on getting as many people vaccinated as possible.

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"We need to pivot the conversation away from thinking of herd immunity as a target we get to or we don't," Lauren Ancel Meyers, a professor of statistical and data science and director of the Covid-19 Modeling Consortium at the University of Texas at Austin, said. "It's simple—the more immunity, the better off we'll all be."

Similarly, Fauci said at a White House briefing, "Rather than concentrating on an elusive number, let's get as many people vaccinated as quickly as we possibly can."

Accordingly, several federal and state initiatives have launched to maximize vaccine acceptance and meet people where they are at—physically and mentally—to talk about vaccination, the New York Times reports.

For instance, HHS earlier this month announced the Covid-19 Community Corps, a volunteer group aimed at spreading awareness about the safety and efficacy of vaccines in their communities. And President Biden this week is expected to unveil a plan aimed at encouraging vaccination, calling on employers to cover the cost of employees taking time off work to get their shots.

Meanwhile, state health officials in Louisiana have started planning small vaccination events in "places that people feel comfortable," Joseph Kanter, the state's health officer, said, ranging from door-to-door visits in neighborhoods to Buddhist temples, truck stops, and the docks where commercial fishermen work.

In North Dakota, employers are partnering with state government to bring Covid-19 vaccines to workplaces. In New Orleans, the city partnered with a bar in a "shots for shots" promotion, and in Alaska's Kenai Peninsula, EMS personnel are bringing vaccines to any residence or business with more than three people.

"It's really going to be all about the ground game," Bechara Choucair, White House vaccinations coordinator, said. "It's going to be about planning at the local level. It's going to be about microplans. It's going to be about county by county, ZIP code by ZIP code, census tract by census tract to make sure what are the strategies that work" (Weise, USA Today, 4/20; Goldberg/Roubein, Politico, 4/21; Owens, "Vitals," Axios, 4/21; Stolberg/Karni, New York Times, 4/21; Fast, "Shots," NPR, 4/20; Diamond, Washington Post, 4/20).