Sarah Zhang writes:

Let’s begin by defining our terms. Herd immunity is the hazy, long-promised end of the pandemic, but its requirements are quite specific. Jennie Lavine, a biologist at Emory University, likens it to wet logs in a campfire. If there’s enough water in the logs—if there’s enough immunity in a population—“you can’t get the fire to start, period,” she says. To be more technical about it, a population reaches herd immunity when the average number of people infected by a single sick person falls below one. Patient zero might infect another person, but that second person can’t infect a third. This is what happens with measles, polio, and several other diseases for which vaccines have achieved herd immunity in the United States. A case might land here, but the spark never finds much dry fuel. The outbreak never sustains itself.

For COVID-19, the herd-immunity threshold is estimated to be between 60 and 90 percent. That’s the proportion of people who need to have immunity either from vaccination or from prior infection. In the U.S., the countdown to when enough people are vaccinated to reach herd immunity has already begun.

But what if we still can’t get the logs wet enough? What if they are drying out faster than we can douse them? A number of signs now point to a future in which the transmission of this virus cannot be contained through herd immunity. COVID-19 will likely continue to circulate, to evolve, and to reinfect. In that case, the goal of vaccination needs to be different.

While COVID-19 vaccines are very good—even unexpectedly good—at preventing disease, they are still unlikely to be good enough against transmission of the virus, which is key to herd immunity. On the whole, we should expect immunity to be less effective against transmission than against disease, to wane over time, and to be eroded by the new variants now emerging around the world. If vaccine efficacy against transmission falls below the herd-immunity threshold, then we would need to vaccinate more than 100 percent of the population to achieve herd immunity. In other words, it becomes downright impossible. [Continue reading…]