Trump administration officials are starting to implement policies that suggest a “herd immunity” strategy – a controversial approach that involves deliberately allowing the coronavirus to spread to build up population resistance more quickly, while protecting the most vulnerable.

In theory, as the number of survivors with immunity increases to a certain level, the virus’s spread would slow and eventually stop. The only problem: A whole lot of people would die before that point.

At a news briefing last week, World Health Organization officials called pursuing such a herd immunity strategy “very dangerous.”

“If we think about herd immunity in a natural sense of just letting a virus run, it’s very dangerous,” said Maria Van Kerkhove, the WHO’s technical lead on the COVID-19 pandemic. “A lot of people would die.”

Soumya Swaminathan, WHO’s chief scientist, said focusing on controlling transmission through public health measures while scientists develop vaccines should be the primary strategy. She pointed out that “there really hasn’t been any infectious disease that has been controlled just by allowing natural immunity to happen.”

The United Kingdom pursued such a strategy early on but abandoned it when officials saw the consequences. Sweden, which pursued a similar strategy, has been heavily criticized by public health officials and infectious-disease experts as reckless – the country has among the highest infection and death rates in the world.

But the idea of “herd immunity” continues to get attention in some quarters: Conservative television host Laura Ingraham has tweeted that pursuing herd immunity was the “only practical way forward.”

How exactly does herd immunity work?

Herd immunity occurs when enough people become immune to a disease that the virus has trouble transmitting to new hosts and can eventually stop. The two main ways to reach herd immunity are through natural immunity – when you get the virus and your body develop antibodies that protect it against future infection – or through vaccinations.

Depending on the virus, there is a certain percentage of population that needs to achieve immunity before herd immunity can be reached – called the “herd immunity threshold.”

For example, measles – an especially contagious disease – only slows down after about 95% of people become immune.

It remains unclear how large a portion of the population must become infected with the new coronavirus to reach that threshold. Estimates have ranged from 20% to 80%. As scientists have learned more about the virus, some have narrowed their estimates to between 40 and 70%.

Swaminathan said that given the coronavirus’s transmissibility, it is likely about 65 to 70% of the population would need to become infected for there to be herd immunity.

Getting to such a threshold would “take a very long time,” she said, noting that seroprevalence surveys that measure people’s antibodies to the virus show that on average only 5 to 10% of people around the world have them. That percentage is higher in some cities such as New York City, where many have been infected. (New research suggests that people without antibodies may nonetheless have T-cell immunity from earlier contact with other coronaviruses, but that has not been proved.)

Swaminathan said those low rates of antibodies in most places mean that “a majority of the world’s population currently is still susceptible to this virus, which means that the infection can go on and on and on in waves, and so the best way to achieve that kind of population immunity would be through a vaccine.”

Why is pursuing herd immunity strategy deadly business?

Proponents of herd immunity talk of segregating and thereby protecting seniors, nursing home residents and others most likely to die of the virus, while allowing the virus to spread among the young. But growing evidence shows that young people – who work outside the home, or who surged into bars and restaurants when states relaxed shutdowns – are infecting their more vulnerable elders, especially family members.

Such a strategy is also complicated in America by the fact that a larger portion of young and middle-aged people here have higher rates than elsewhere of obesity, heart and lung disease and other health issues that make them more likely to have serious or lethal cases of the disease.

Exactly how many people would die before a population can reach immunity depends on a number of complicated variables, but even rudimentary, back-of-the-envelope calculations show it is likely to be substantial.

With a population of 328 million in the United States, it may require more than 2 million deaths to reach a 65% threshold of herd immunity, assuming the virus has a 1 % fatality rate, according to an analysis by The Washington Post. Even if both the herd immunity threshold and the fatality rate proved to be toward the lower end of current estimates – with 40% needing to be infected and a 0.5% fatality rate – the country could still expect 656,000 deaths to achieve herd immunity, nearly four times as many as the country has already suffered.

It also remains unclear whether people who recover from COVID-19 have long-term immunity to the virus or might become reinfected. That could complicate how herd immunity may be achieved. Scientists are still learning exactly who is vulnerable to the disease. From a practical standpoint, it is nearly impossible to sufficiently isolate people at greatest risk of dying from the younger, healthier population, according to public health experts.

Harvard epidemiologist Marc Lipsitch points out that even when a community, or a nation, gets to herd immunity, it will still have cases. “It doesn’t mean cases stop,” he said. “It just means they start to slow down.”

“We’d all like to know how far we are from the end of this,” Lipsitch said. “But no matter what the exact threshold, it’s largely an academic question because we’re nowhere near herd immunity.”

“No wishful thinking is going to make this go away,” said Michael Osterholm, director of University of Minnesota’s Center for Infectious Disease Research and Policy. “We can’t wait for herd immunity. We can’t even wait for a vaccine. We need to be doing everything we can now to reduce transmission. We need plans and national action. We need vision. This isn’t something we can keep kicking down the road. We can’t keep talking about next month or two months from now. We need action now.”

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The Washington Post’s Yasmeen Abutaleb and Harry Stevens contributed to this story.

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