Estimates vary, but scientists believe the Delta variant is perhaps two to three times more transmissible than the original virus that first emerged from China in 2019. In the absence of any mitigating public-health measures, every 10 people infected with the original virus would go on to infect 25 people, on average. With Delta, 10 infections would likely result in between 60 to 70 new infections, a big increase.

That puts the threshold for so-called herd immunity, or what some scientists prefer to call community protection or population immunity, further out of reach. Where previously governments and public health officials had hoped herd immunity might have been achievable with between 60% and 70% of the population vaccinated, Delta means that threshold is likely between 80% to 90%, said Mark Woolhouse, professor of infectious disease epidemiology at the University of Edinburgh.

Some estimates put the threshold higher still, at more than 95%, once vaccines’ less than 100% effectiveness at reducing viral transmission has been taken into account. On top of that, there is uncertainty over how long the protection afforded by vaccines lasts or if new variants may emerge that can sidestep such defenses.

“Because the herd immunity bar has been raised so high, it is not a question of when we will get there, but if we will get there,” said Prof. Woolhouse.