Scientists widely believe that coronavirus will not die off in the U.S. until between 50% and 70% of Americans have either become infected with the virus or been inoculated against it through a vaccine.

With a vaccine not expected for another 12-18 months, epidemiologists and some government officials see a particularly bleak path to the end of the pandemic, which has already claimed more than 27,000 lives in the U.S.

“The prospects of mass gatherings is negligible at best until we get to herd immunity and a vaccine,” California Gov. Gavin Newsom said at a press conference Tuesday.

“Several rounds of social distancing will be required to get us to ‘herd immunity’ in the absence of vaccination,” Dr. Marc Lipsitch, a Harvard epidemiologist, said at a separate press conference announcing the results of a study of the virus.

Herd immunity has proved to be a controversial topic during the coronavirus outbreak since, according to some estimates, nearly 2 million Americans would die on the way to achieving the threshold required to end the spread of the virus.

The debate among those who view herd immunity as inevitable has also splintered over whether it should be actively embraced as a strategy or delayed for as far long as possible.

The two competing camps are largely divided over the lethality of coronavirus (estimates range from 0.1% to 3.4% fatality rates) and whether an extended period of social distancing measures to delay onset of herd immunity is worth the destruction it will bring the economy.

The debate is further complicated by limited research on just how much of the U.S. population has already been infected during the pandemic.

As scientists search for answers to those questions, the camp of health experts advocating for slowing transmission of the virus — or “flattening the curve” — has won control of policy responses to the pandemic.

President Donald Trump reportedly brought up the possibility of allowing coronavirus to “wash through” the country, establishing herd immunity, during a coronavirus task force meeting last month. Dr. Anthony Fauci, the leading infectious disease expert on the White House coronavirus task force, reportedly pushed back on the idea, saying “many people would die.”

But the task force, and the Trump administration as a whole, has been ambiguous on what the end game for the pandemic will actually look like.

The administration has focused on getting through the first wave of the pandemic and setting up widespread testing and the capacity to trace infections to limit future outbreaks.

The strategy has required business closures and widespread stay-at-home orders that have obliterated the economy. Administration officials have said they expect a resurgence of cases later in the year, but have not said what they expect to happen in 2021.

Fauci and his fellow task force scientist, Dr. Deborah Birx, have not publicly discussed whether herd immunity is the ultimate end to the pandemic. Reporters at the task force’s daily press briefings have also not asked whether herd immunity is likely to be reached before development of a vaccine.

That ambiguity stands in stark contrast to a sober assessment offered up by some epidemiologists — and Newsom — who see herd immunity as all but inevitable.

In March, Newsom told Trump that the state projects that 56% of California residents — or 25.5 million people — would become infected with coronavirus over the next eight weeks.

Dr. Michael Osterholm, an infectious disease expert at the University of Minnesota, also accepts herd immunity as an inescapable reality of the pandemic given how far off a vaccine is.

“Covid-19 will go away eventually in one of two ways,” he wrote in a March 21 op-ed for the Washington Post. “Either we will develop a vaccine to prevent it, or the virus will burn itself out as the spread of infection comes to confer a form of herd immunity on the population. Neither of those possibilities will occur quickly.”

Osterholm has estimated that 1.6 million Americans will die from coronavirus. That’s based on an assumption that half of the U.S. population will become infected, and one percent of those will succumb to COVID-19.

The gist of a Harvard report is that the best strategy for public health officials and policy makers is to slow the spread of the virus in order to allow the health care system to process sick patients.

Until then, the researchers argue, the American public should engage in repeated waves of lockdowns as outbreaks emerge in their communities. They predict that lockdowns will be needed until 2022.

That type of scenario presents a quandary for policymakers who have to balance protecting the public from coronavirus while also preventing the economy from completely falling apart. More than 17 million Americans have filed for unemployment in less than a month since social distancing guidelines were put in place.

On the other end of the pandemic spectrum from the Harvard researchers of the world are a group of scientists calling for a more proactive approach to herd immunity. Rather than deliberately slowing transmission of the virus through economic lockdown, that group supports a strategy of allowing the virus to transmit among people who are likely to avoid serious illness.

At the same time, the elderly and people with underlying health conditions should be isolated while the virus infects people less likely to suffer complications from COVID-19.

Dr. Matt Strauss, a Canadian professor, has called for achieving herd immunity “in a controlled and strategic manner.”

He’s suggested allowing younger people to return to the workforce as long as they do not live with people most vulnerable to the virus. The idea behind the strategy is that, if 60% of the population must become infected in order to achieve herd immunity, it’s best to expose those most likely to recover from the disease.

Proposals like Strauss’s have been widely condemned as they are seen as promoting infection rather than avoiding it. But making the debate particularly murky is the position held by epidemiologists who view coronavirus as extremely lethal, but expect the pandemic to end only through natural herd immunity.

Researchers are unsure just how close the population is to achieving herd immunity. The proximity to herd immunity is also inconsistent across the country.

New York City, the epicenter of the pandemic in the U.S., is likely far closer to the herd immunity threshold than a town in rural Kansas, which has likely avoided widespread infection.

A study of pregnant women in New York City found that 13% were infected with coronavirus and didn’t know it. While still far short of the herd immunity threshold, that rate of infection is likely far higher than the rest of the U.S.

Dr. Scott Gottlieb, the former commissioner of the Food and Drug Administration, estimated that between 2% and 5% of Americans have been infected with the virus.

Perhaps the most important unanswered question regarding immunity is whether recovered patients are protected against reinfection. Most infectious diseases confer some immunity to recovered patients, so health experts believe the pattern will hold for coronavirus.

But studies out of China and South Korea have showed that some patients have tested positive for coronavirus after they were thought to have recovered. Other patients have displayed low levels of antibodies following recovery, raising the possibility that they have limited protection against reinfection.

In that worst-case scenario, a vaccine would be the only hope to end the pandemic.