Less than 15 percent of Americans have been infected by the virus that causes covid-19. If immunity among those who have been infected and survived is strong and long-lasting (and it may well be neither), and if herd immunity kicks in at 60 percent infection of the population (and it might be higher), with a fatality rate of 0.5 percent among those infected, then at least another half-million Americans — in addition to the 220,000 who have already died — would have to die for the country to achieve herd immunity. And that’s the best-case scenario. The number of deaths to get there could be twice as high.

The route to herd immunity would run through graveyards filled with Americans who did not have to die, because what starts in young adults doesn’t stay in young adults. “Protecting the vulnerable,” however appealing it may sound, isn’t plausible if the virus is allowed to freely spread among younger people. We’ve seen this in families, communities and entire regions of the country. First come cases in young adults. Then the virus spreads to older adults and medically vulnerable people. Hospitalizations increase. And then deaths increase.

The vulnerable are not just a sliver of society. The 65-and-over population of the United States in 2018 was 52 million. As many as 60 percent of adults have a medical condition that increases their risk of death from covid-19 — with many unaware of their condition, which can include undiagnosed kidney disease, diabetes or cancer. The plain truth is that we cannot protect the vulnerable without protecting all of us.