Going out, getting sick, and hastening herd immunity seems to be bubbling up as an alternative to taking all-too-familiar steps in the name of buying time until a vaccine finally subdues the coronavirus pandemic.

That’s a bad idea, though it’s easy enough to understand its appeal.

The metaphorical angel on one shoulder points to the realities of exponential viral growth and a consensus of infectious-disease specialists and epidemiologists: COVID-19 is highly contagious; the vast majority of us enjoy no immunity to it; health care institutions will, if the coronavirus is left unchecked, be overwhelmed; and people who could have been saved will die. That angel whispers, “Stay put. Wash hands. Wear the mask. Keep your distance. Avoid crowds.”

On the other shoulder, the devil appeals to our stir-craziness; our unemployment numbers; our desire for human contact; our frustrations of lost sports competitions, graduations, and vacations; and to the uncertain prospects about vaccinations. “Go out. Get to work. Have fun again!” that second voice beckons.

That devil is suggesting, indirectly, that if enough of us get just on with our lives, fall ill, and recover, we’ll develop herd immunity the good old-fashioned way. That’s opposed to developing herd immunity the modern-medicine way, which is through a vaccine that spurs our bodies to recognize the coronavirus without having to fall ill and recover.

Either way, herd immunity requires that enough of the population is immune that the disease can’t keep spreading even if some people remain vulnerable. While that number varies depending on the disease, it’s big. With measles – highly contagious – a population needs to achieve about 95% immunity to achieve herd immunity. With COVID-19, no one knows for sure yet, but best estimates fall in the 60% to 70% range. (FiveThirtyEight built a herd-immunity calculator to show how this works. It accompanies an article titled “Without A Vaccine, Herd Immunity Won’t Save Us.”)

Some numbers on herd immunity

Let’s ignore the possibility that COVID-19 is much more contagious than we think it is and go with a 60% herd-immunity threshold. The numbers for herd immunity still aren’t good, and numbers matter with a disease that spreads exponentially.

Sixty percent of the U.S. population of 327 million is about 196 million people. As of mid-August,  the number of confirmed U.S. cases was approaching 5.15 million – way short of herd immunity, even if one assumes that undiagnosed COVID-19 illnesses dwarf those that have been confirmed through testing (best guesses are that the actual case count is five to 10 times higher).

Simple arithmetic can help us understand how getting to herd immunity would push the health care system beyond the brink – the brink being the estimated 84,750 ICU beds in the United States. Let’s assume that 0.2% of all symptomatic coronavirus cases are fatal (this is the rosiest of the CDC’s latest estimates, that all coronavirus fatalities are first sent to an ICU, and that ICUs are able to save 75% of COVID-19 patients. Let’s also say 5% of the U.S. population is already immune – that’s about the rate in Spain, which has been hit similarly hard by the disease, adjusted for population.

With those assumptions, getting to 60% herd immunity could cost about 234,000 additional lives in this country. Along the way, it could send about 935,000 people to ICUs. Assuming COVID-19 patients filled every ICU bed in the country (that is, no heart attacks, no strokes, no auto accidents, and so on), and that we could fine-tune our pursuit of herd immunity to precisely calibrate ICU patient loads to available capacity, we would need to endure about three months of maxed-out ICUs, assuming an average stay of eight days.

Such precise ICU load calibration can’t be done for several reasons: COVID-19’s long incubation period; the number of asymptomatic but contagious cases; the lack of clarity in the nature of disease transmission; the uncertain future of social-distancing measures as the country opens up; and the dearth of testing, contact tracing, and quarantining. We would likely blow right past ICU capacity as we unflatten the curve and people who might otherwise live would die.

How about in Colorado? The numbers are smaller but the proportions similar. We’d need about 3.5 million of our 5.8 million people to contract and recover from COVID-19 to achieve 60% herd immunity; another 8,300 could die – seven to eight times the number of deaths so far, depending on how one counts – and we could send more than 16,600 critical patients to ICUs. Perfectly calibrated, we could fill all 1,800 critical-care beds for about two-and-a-half months – again fanciful given the realities of the SARS-CoV-2 virus.

“I think too many people will die if health-care capacity is exceeded,” said Dr. Jonathan Samet, dean of the Colorado School of Public Health and the leader of the Colorado COVID-19 Modeling Group. “Most of us are still at risk. So if we said, ‘Let’s get it over with,’ I think, with reasonable certainty, we’d have thousands more deaths in Colorado and the country, and we’d have catastrophe. That’s why we have to have a very measured lightening of distancing measures, as much as people don’t like it – including me.”

All this arithmetic assumes that contracting and recovering from COVID-19 confers complete immunity over the period of time it takes to get to herd immunity – otherwise, people could get re-infected and infect others. Disease recovery might provide such a shield, and it might not. Nobody knows, though so far, so good.

The Swedish example of herd immunity

What about Sweden? Sweden is going for herd immunity, with open cafes and restaurants and masks nary to be seen. It seemed like it was working: On April 26, the Swedish ambassador to the United States told NPR that about 30% of Stockholm’s population had “reached a level of immunity.” Turns out the figure was more like 7.3% at the time. For that small step toward herd immunity, Sweden has seen about 50% more per-capita cases than its Scandinavian neighbors a 27% jump in excess deaths, far higher than surrounding countries that took much larger social-distancing steps. Despite encouraging senior citizens to stay at home – just as Colorado is doing under its new “Safer at Home” rules – half of Sweden’s coronavirus deaths have been in nursing homes. Safely sequestering seniors and those especially vulnerable to serious COVID-19 consequences is much easier in theory than in practice.

There would be a flood of serious coronavirus cases anyway.

“Our own data for Colorado show that, of people receiving critical care, about 50% are under 60,” Samet said. “So it’s – and I’m in that older age group – it’s not just that we’re going to ‘harvest’ our elders, which I don’t think we want to do. We’d have lots of young people who are sick as well. We’re in this for the long run unless there’s some surprise with the virus.”

As frustrated as we all are with the disruptions and pain the coronavirus has brought, attempting herd immunity without a vaccine looks to be a losing proposition, and one that would bring needless death and misery – especially considering the apparent progress of global efforts to develop vaccines in record time.

The devil is in the details with herd immunity, and those details spell out a clear message: beware of the devil.