The race between the evolution of the virus and the vaccination of human beings boils down to a math problem. On average, everyone infected with the original coronavirus that emerged in Wuhan, China, infected 2.5 other people. Epidemiologists calculated that by vaccinating 70% of the population, that could drop to less than one new person infected, causing the virus to dwindle away.
People infected with the Delta variant, by contrast, appear to infect more people — estimates range from 3.5 to seven new infections. That raises the bar for herd immunity to as high as 85% of the population.
At its current rate, the U.S. won’t hit that goal until December. But 11% to 14% of Americans say they don’t want to be vaccinated if they have a choice. Add that to 10% who want to “wait and see,” and herd immunity in the U.S. looks out of reach.