The world may never reach herd immunity against Covid-19

As Covid-19 surged last year, governments worldwide touted the hope of "herd immunity,” a promised land where the virus stopped spreading exponentially because enough people were protected against it. That’s now looking like a fantasy.

The thinking was that the pandemic would ebb and then mostly fade once a chunk of the population, possibly 60% to 70%, was vaccinated or had resistance through a previous infection. But new variants like delta, which are more transmissible and been shown to evade these protections in some cases, are moving the bar for herd immunity near impossibly high levels. Delta is spurring widening outbreaks in countries like the U.S. and U.K. that have already been walloped by the virus, and presumably have some measure of natural immunity in addition to vaccination rates of more than 50%. It’s also hitting nations that have until now managed to keep the virus out almost entirely, like Australia and China.