Most people understand immunity to mean that once a person has been exposed to a disease, they can’t get it again. It’s an easy concept to grasp, and some people have hoped that widespread immunity could be the way out of this pandemic: If enough of the population becomes immune to the disease, the spread would be stopped, since the virus would run out of new, susceptible targets. The “herd” of immune people would protect everyone.
Fivethirtyeight has built a simple simulation of how the theoretical herd immunity would work. Then the authors conducted several interviews with epidemiologists to figure out what the “cost” of herd immunity would be.
So let’s go back to that 70 percent herd immunity threshold. If the fatality rate is around 0.5 percent and 70 percent of Americans have to get sick before their immunity starts protecting others, that means more than 1.1 million people would die. In New York, even having 21 percent of the population exposed, if that serological survey is accurate, has overrun hospitals and led to the death of one in every 400 New Yorkers, while the vast majority of the population remains susceptible.
“That’s the cost of getting to 20 percent,” said Emma Hodcroft, a postdoctoral epidemiology researcher at the University of Basel in Switzerland. “It really illustrates the price you’re going to pay if you want to get up to the 60 percent or 70 percent that you’ll need for herd immunity, and I hope it really illustrates why that just isn’t a feasible plan.”