For COVID-19, we’re still not certain what the R0 is, so we don’t yet know what the herd immunity threshold is. For now, it’s estimated to be anywhere from 70 to 90 percent. But here’s the problem: To reach even the lower end of that range naturally in the U.S. — imagine giving up on any interventions and just letting the disease run its course — 230 million Americans would eventually become infected and, depending on the fatality rate (more on that later), millions could die.
And reaching herd immunity as the outbreak is raging is a completely different scenario than, for example, doing so after a vaccine has been created. With a vaccine, you can immunize people before they’ve encountered the virus, so by the time the virus gets to a vaccinated population, it has nowhere to spread. But in an active outbreak, even once the herd immunity threshold is reached, the infection keeps going. It takes time for the spread of the disease to crest because, for a while at least, many contagious people will each still infect a small number of vulnerable people, which means even more people will get sick and die. It’s a phenomenon known as “overshoot.”
“The disease sort of stops increasing at the point when you reach herd immunity, but there’s still lots and lots of people infected. It only slowly goes down, and on its way down, [it] infects another third of the population,” said Richard Neher, an evolutionary biologist at the University of Basel in Switzerland. “If you drive a car and suddenly you switch off the engine, it doesn’t stop instantly.”
Join the conversation as a VIP Member